The current views on general Philosophy have usually exercised a considerable influence upon the course of development of scientific thought; and this influence has been reciprocal. The strength of this connection was in former times increased by the fact that one and the same man not infrequently combined the functions of Philosopher and man of Science. The cases of Aristotle Descartes Bacon and Leibniz may be cited as instances of this fact. Even Kant had strong claims to be considered as a man of Science. Philosophers such as Locke Hume and Berkeley were largely occupied with the discussion of matters which have a close connection with the epistemology of the Natural Sciences. Not only the more systematized metaphysics of professed Philosophers but also the more popular and less systematized ideas on Metaphysics current at a particular time had a marked influence upon the forms in which the scientific conceptions of the time found expression and exercised a considerable directive power upon the lines of scientific investigation. Popular thought is at all times permeated by metaphysical ideas however detached and unsystematized their form may be and however unconscious the majority of men may be of their presence and true character. I have already alluded to the strong tendency which Natural Science has exhibited during the last two centuries to extend its original scope and to transform itself into an all-embracing World-Philosophy.
Natural Science in Relation to Philosophy
With a view to the delimitation of the domain of Natural Science in relation to human experience as a whole it is necessary to make an attempt to clear up ideas as regards the position of Natural Science relative to some of the various systems of Metaphysical Philosophy with which it has at various times found itself in contact. Fortunately it is I believe possible to do this without considering in much detail the characteristics of the various metaphysical systems which have in different ages arisen and some of which were held contemporaneously by different schools of philosophical thought. For our purpose it will suffice briefly to refer to a very few of the more fundamental points in which these systems with their innumerable points of difference in less fundamental respects differ from one another. In particular it is requisite to attempt to draw some conclusion as regards the answer that should be given to the question what ontological assumptions are necessary for the special purposes of Natural Science. The history of Science shows that assumptions of this kind have in fact played a large part sometimes in promoting and sometimes in hindering progress. But the most important question we have to consider is how far such assumptions are indispensable for the existence and efficiency of Natural Science as an organized system devised for the systematic ordering of our physical percepts in whatever degree experience may show that this ordering is possible. It is clear that in accordance with sound Methodology all unnecessary assumptions should be discarded as unessential to Natural Science even when they have in point of historical fact been employed as part of the scaffolding used in the building up of the various departments of the domain. The existence and successful functioning of Natural Science cannot properly be invoked as a decisive reason for accepting any ontological theory or assumption which cannot be shown to be an indispensable element in scientific thought considered as a coherent body of doctrine.
In the subject-object relation which is fundamental in all experience although the phenomena with which Natural Science concerns itself are objects for some subject the precise nature of the activity of the subject as a factor in the relation is irrelevant to Natural Science in the sense in which the term is here used. The investigation of this activity falls within the domain of Psychology which for reasons I shall presently explain is not part of Natural Science as here delimited. For Natural Science the role of the subject in perception may provisionally or methodologically be regarded as purely passive that of experiencing physical perceptions. Thus Natural Science takes these perceptions as simple data and need not attempt to give any account or how far the activity of the subject is necessary for their existence or how far they depend upon something foreign to the percipient. On such philosophical and psychological matters Natural Science as such has nothing to say and may take up a neutral attitude in relation to conflicting views concerning them. It should however be observed that the formation of concepts must be regarded as due to the psychical activity of subjects although Natural Science is not concerned with any detailed investigation of the modes and laws of such psychical activity. This form of psychical activity is considered methodologically as distinct from any activity or acts of attention connected with the reception of percepts; but Natural Science is committed to no assumption that such separation of psychical function is valid in an ultimate sense or that it is necessarily anything more than one of those kinds of separation which discursive thought is compelled to make of elements which from a more fundamental point of view may come to be regarded as inseparable elements of an essential whole.
What we call a material object is prima facie a construct built up by the synthesis of a group of actual sense-impressions and of images of earlier sense-impressions stored up in the memory. An effect of the memories of sense-impressions is to enable us in perceiving an object to dispense with some of the actual sense-impressions which make up the construct. Thus when we see a stone we know that it is hard without verifying the fact by touch in each instance; this is due to the memory of earlier sense-impressions and to a belief in their unchangeableness. This view of the nature of a material object implies that there is a percipient for whom the object is a percept; and this involves the subject-object relation. The question now presents itself can the object be properly regarded as existent when there is no percipient for whom it is a percept? In other words can the object be separated out from the subject-object relation and be regarded as having an existence independent of any percipient and of all percipients? Is there any definite meaning and if so what to be attached to the assertion of such independent existence? The kind of answer which has been given to such questions varies greatly in different philosophical systems. For us the importance of the matter depends upon whether it makes any essential difference to Natural Science what answer is given to these questions and to other ontological questions relating to the existence of the percipient or subject. The answer which I suggest and advocate is that for the special purposes of Natural Science it is immaterial what answer is given to these questions; that in fact ontological hypotheses or theories on these points are irrelevant to Natural Science; that it is sufficient for example for the purposes of Natural Science to regard a perceptual object as a construct of sense-impressions whatever else it may for the purposes of systematic Philosophy be regarded as being or implying. But this answer is one which would not have been given until recent times by any man of Science and probably would not be given at the present time by the majority of men of Science. A material object has most frequently been regarded as having some kind of sub-stratum not identical with the synthesis of sense-impressions but as a thing in itself a kind of bearer of the various properties or qualities such as extension motion hardness colour elasticity etc. which are regarded as giving rise to the component sense-impressions. Thus material objects are regarded as having an entity material substance as their essential foundation; the various properties or qualities being regarded as inherent in the substance. The real object or thing in itself is not identified with any or all of the properties which it possesses and with which our sense-impressions are related; it is not itself directly perceived although its existence is supposed to be a necessary inference from the existence of the percept. This point of view was clearly expressed by Locke who writes1:
When we talk or think of any particular sort of corporeal substances as horse stone etc. though the idea we have of them be but the complication or collection of those several simple ideas of sensible qualities— yet because we cannot conceive how they should subsist alone nor one in another we suppose them existing in and supported by some common subject; which support we denote by the name substance.
This conception of substance as a sub-stratum of matter has been held by the adherents of some realistic systems of Philosophy by many or probably most men of Science and it appears to be held by those who are dominated by the set of notions usually described as common sense or as naive realism. The thing in itself has been regarded as having an existence independent of any and all percipients; thus forming an independent Real. The doctrine of the distinction between substance and its properties or accidents was an essential part of Scholasticism pressed into the service of Ecclesiasticism. When it was pointed out by Locke and others that at least some of the accidental properties of the thing in itself are clearly not independent of the percipient a division was made of the qualities of the substance into two sets called respectively primary and secondary qualities. The primary qualities extension and motion and probably inertia were regarded as inherent in the object itself and thus like the substance independent of any percipient; but this independence was not asserted of the secondary qualities such as colour sound smell and temperature. These last were regarded as essentially dependent on the percipient but also as conditioned by the primary qualities of the substance. The substance or the sub-stratum of all material things is incapable of being directly perceived but is regarded as something which is of necessity conceived by the mind as existing independently of itself as the bearer of qualities just as an adjective requires a substantive which it qualifies. In Leibnizian monadism and also in more modern forms of spiritualistic pluralism matter is regarded as the manifestation of the activity of a plurality of psychical beings; and thus substance is of a psychical character.
The exact opposite of this notion of the necessity of the category of substance as the sub-stratum of the material world was advocated by Berkeley the great English idealist philosopher. With him what we have spoken of as percepts and what he calls ideas constitute the whole reality of material objects. He wrote1:
It is indeed an opinion strangely prevailing amongst men that houses mountains rivers and in a word all sensible objects have an existence natural or real distinct from their being perceived by the understanding.
The view expressed by Berkeley that there exists no sub-stratum or substance independent of perception is characteristic of idealistic Philosophy and is in sharp contrast with the opposed realistic view.
To decide between these opposed opinions is no part of the function of Natural Science; in fact the method which Science employs makes it incapable of doing so. Ex hypothesi substance things in themselves if they exist cannot be directly known as percepts but are inferences from what is perceived; and thus they cannot form part of the perceptual order of things which it is the function of Natural Science to classify and describe. Whether they exist or not is consequently quite immaterial to Natural Science because even on the realistic assumption. Science is actually concerned only with the percepts to which things in themselves give rise and not with the real world which the realistic Philosopher takes to exist behind phenomena. As between these two divergent philosophical views position of Natural Science may be taken to be that of a neutral. Natural Science is not compelled either to affirm or to deny the existence of real substance or of monads as independent reals. It has no need to take account of the category of real substance. The history of Science is full of assumptions of the existence of ethers and substances of various kinds; but these may be taken to be concepts not realities independent of the psychical world. They are parts of the conceptual scaffolding which go to build up scientific theories and laws.
In speaking of the function of Mathematical theories Poincaré has given1 a description of their function which might I think be extended to scientific theories in general. He writes:
Mathematical theories have not as their object to reveal to us the real nature of things; that would be an unreasonable claim. Their sole aim is to coordinate the physical laws that experience reveals to us but which without the aid of Mathematics we could not even enunciate.
Even to-day men of Science under the influence of philosophic Realism or of the unsystematized Philosophy of common sense are extremely reluctant to abandon the idea that the notion of real entities something more than concepts is essential to Natural Science. Thus for example Professor A. N. Whitehead writes1:
Another favourite solution the most attenuated form which the bifurcation theory assumes is to maintain that the molecules and ether of Science are purely conceptual. Thus there is but one nature namely apparent nature and atoms and ether are merely names for logical terms in conceptual formulae of calculation. But what is a formula of calculation? It is presumably a statement that something or other is true for natural occurrences. Take the simplest of all formulae. Two and two make four. This—so far as it applies to nature—asserts that if you take two natural entities and then again two other natural entities the combined class forms four natural entities. Such formulae which are true for any entities cannot result in the production of the concepts of atoms. Then again there are formulae which assert that there are entities in nature with such and such properties say for example with the properties of the atoms of hydrogen. Now if there are no such entities I fail to see how any statements about them can apply to nature... The current answer is that though atoms are merely conceptual yet they are an interesting and picturesque way of saying something else which is true of nature. But surely if it is something else that you mean for heaven's sake say it. Do away with this elaborate machinery of a conceptual nature which consists of assertions about things which don't exist in order to convey truths about things which do exist. I am maintaining the obvious position that scientific laws if they are true are statements about entities which we obtain knowledge of as being in nature; and that if the entities to which the statements refer are not to be found in nature the statements about them have no relevance to any purely natural occurrences. Thus the molecules and electrons of scientific theory are so far as science has correctly formulated its laws each of them factors to be found in nature. The electrons are only hypothetical in so far as we are not quite certain that the electron theory is true. But their hypothetical character does not arise from the essential nature of the theory in itself after its truth has been granted.
If I understand this passage aright the ideas of the nature and functions of a scientific theory expressed in it are in fundamental opposition to those which I here advocate. Whatever else they are molecules and electrons are concepts and even Dr Whitehead is not quite sure that they are anything else. They are concepts in scientific theories and the only question about a scientific theory is not whether it is in Dr Whitehead's sense “true” but whether it is logically coherent and how far it is adequate for the purpose of representation; whether it will be superseded by some other theory which employs other concepts because that theory is applicable to a greater range of phenomena than the theory which employs molecules and electrons. We do not perceive in Nature entities such as atoms and ether; we do not perceive entities at all if an entity be taken to be anything more than a construct of a complex of sense-impressions present and past. What Dr Whitehead speaks of as an elaborate machinery of a conceptual nature which consists of making assertions about things which don't exist is really a scheme involving things which do exist as concepts; and conceptual atoms have been employed because they have been found not merely “interesting and picturesque” but because at all events for the time being they have been found to be the best available means for representing what “is true in nature” in Dr Whitehead's phraseology; in the sense that they resume conceptually part of what we perceive as natural phenomena. Natural Science postulates as a working hypothesis only that the perceptual complex is such that tracts of it are capable of conceptual description by scientific schemes. It does not require any postulate as to detailed systems of relations or of entities within that perceptual complex or within any supposed reality behind that complex which shall account for the fact that the working hypothesis has proved successful.
In speaking of the new conceptual scheme of space-time which is associated with the name of Einstein Sir Oliver Lodge writes1:
In such a system there is no need for Reality: only phenomena can be observed or verified: absolute fact is inaccessible. We have no criterion for truth; all appearances are equally valid; physical explanations are neither forthcoming nor required; there need be no electrical or any other theory of the constitution of matter. Matter is indeed a mentally constructed illusion generated by local peculiarities of space.
And again he writes concerning the same theory:
But notwithstanding any temptation to idolatry a physicist is bound in the long run to return to his right mind; he must cease to be influenced unduly by superficial appearances impracticable measurements geometrical devices and weirdly ingenious modes of expression; and must remember that his real aim and object is absolute truth however difficult of attainment that may be that his function is to discover rather than to create and that beneath and above and around all Appearances there exists a universe of full-bodied concrete absolute Reality.
The notion indicated by Sir Oliver Lodge that it is the function of Natural Science to search for reality behind physical phenomena still receives support from the ranks of Philosophers. Thus for example Professor H. Wildon Carr writes2:
The modern era of philosophy from Descartes onwards has been dominated by the insistence of the scientific problem—that is the problem of the ultimate nature of the reality we study in physical science by the experimental method.
These examples may suffice to illustrate the persistence of this view which I believe to be erroneous even at the present time.
It is interesting to observe that some men of Science who avowedly regard Metaphysics with distrust and aversion are most insistent in claiming “reality” for material objects and for molecules atoms and ether; oblivious of the fact that they are as it would seem unnecessarily by using the term “reality” illustrating the fact of the extreme persistence in thought of metaphysical conceptions of the theoretical and of the common sense order. The term “reality” has an intensely metaphysical complexion; and its shades of meaning in different philosophical schemes vary considerably. Many thinkers like Plato have held that the domain of concepts has a claim to “reality” superior to that of the perceptual world.
I have already spoken of the dualism which recognizes in the world two distinct domains the psychical and the physical; or as is sometimes said with less accuracy mind and matter. The ultimate relation of these two domains is for the Philosopher to elucidate. For the man of Science this dualism should be regarded only as provisional and methodological and need not be taken to involve any final assumption of the fundamental disparateness or separation of the two domains. Mind and Matter have frequently been regarded as ultimately reducible to one of the two or to some third basal existent more fundamental than either of them; but Natural Science so long as it confines itself to its own original functions does not stand or fall with any hypothesis of this character. A monistic system of Philosophy whether it be of the kind called materialistic or of the kind described as spiritualistic or whether it be described as neutral will no doubt make the attempt to interpret scientific theories in terms appropriate to the particular system. Pluralistic schemes of Philosophy such as the Monadology of Leibniz or other more modern forms of Pluralism must also be left to find their own modes of interpretation of Natural Science. But scientific thought leading to what is known as positive knowledge needs none of the ontological assumptions peculiar to any one of the many rival philosophical theories of the nature of reality.
The naïve realist and some philosophers regard the objects of perception such as we perceive in our normal life when awake as still existing very much as we perceive them whatever that may mean even when no sentient being is actually perceiving them. This is also the view of common sense; but scientific knowledge in accordance with the view of its nature that I am here propounding is independent of the acceptance or rejection of this opinion. Scientific knowledge is also independent of the assumptions made in any other form of realistic Philosophy. The pure phenomenalist holds that perceptual objects exist only when and so long as they are being perceived as objects by a percipient; and thus that the only realities are mental states and the objective constructs actually perceived. Modified statements hold good for intermediate philosophical theories as for example the kind of phenomenalism which admits the existence of νοούμενα behind phenomena. The descriptive view of the functions of scientific theories and laws which I here adopt has sometimes been characterized as phenomenalism; but this view does not necessarily involve the acceptance of any philosophical doctrine of phenomenalism. The phenomenalism of Natural Science is methodological only.
That the distinctions which we make in order to meet the necessities of our discursive methods of apprehension necessarily correspond to similar distinctions in an objective reality is an assumption of which though it has been persistent through the whole history of speculative thought Natural Science has no need whatever its merits may be in the view of Philosophers of various schools.
Accordingly the view of the function and limitations of scientific knowledge which has arisen as the result of recent criticism has as one of its implications the assumption that Natural Science is independent of purely ontological theories as applied either to percepts or to percipients. If in the sense employed by any particular philosophical system the existence of real entities be asserted in any particular connection. Natural Science can make no use of such assertion since all scientific theories can be stated in a form which is independent of such ontological assumption. There is a considerable practical advantage in the rejection as unnecessary for the purposes of Natural Science of all superfluous ontological hypotheses. So far as Science can take up the position of neutrality or of detachment with respect to the ever-conflicting and ever-varying ontological views of Philosophers it does not share in the vulnerability which may be held to attach to any of the metaphysical theories that are involved in the conflict. In view of the instability of all definite metaphysical schemes an instability which does not show much sign of disappearing the greater stability with which this position of neutrality endows Natural Science has great advantages.
In making these remarks as to the relation of Natural Science with Metaphysics I have no desire to associate myself with the views of those men of Science who decry Metaphysics as a barren study which can lead to nothing but fruitless sophistries. On the contrary I fully recognize that in all ages Philosophical thought has been indispensable in ministering to one of the deepest and most ineradicable impulses of the human spirit; the imperative impulse to penetrate in some degree into the recesses of the great mystery of life and existence with which we are all in relation. Although no generally accepted solution of even the fringes of that mystery may be in sight yet Philosophy has done much to fix and clarify the form in which fundamental questions can be properly stated and has also rendered great services in definitely removing by its criticisms many irrelevant accretions which had gathered round the main philosophical questions. The attempt to penetrate to a reality in which we may find something permanent and stable in behind or above the impermanent and unstable elements of physical and psychical phenomena is one which will continue to be made as long as the human spirit resembles what it has been in the past and is in the present; although it is no part of the functions of Natural Science to take part in this attempt. The impulse from which this attempt arises will never for long be stifled by merely negative criticism. But progress on the path to reality cannot be made by the simple expedient of transmuting such concepts as those of matter ether atoms electrons into real substances or entities.
Philosophy has in recent centuries rendered valuable services to Natural Science in assisting it to remove various injurious accretions derived from medievalism of supposed a priori knowledge and of unnecessary and irrelevant ontological assumptions. The attention of scientific thinkers has thus been concentrated on phenomena themselves and on the mental representation of them. Science has no longer been hampered by medieval prejudices relating to occult properties of supposed sub-strata behind phenomena or by conceptions of Nature as necessarily conforming to ideas constructed by a priori thought. The advance of Natural Science involves two main factors first the discovery by observation and experiment of facts; and secondly the procedure of reflective thought in classifying groups of facts devising rules or laws to which the groups of facts conform and constructing general theories which symbolize phenomena and under which groups of laws can be subsumed. Both these factors are essential in all genuine Science but the emphasis placed on the two factors has varied at different periods and with different thinkers. At some periods and with some persons there has been a tendency unduly to subordinate one or other of these factors. Among the Greek thinkers Aristotle can be distinguished by the great importance he attached to the observation of facts but he did not thoroughly grasp the nature of the slow and painful process by which empirical results are obtained and used to build up genuine scientific theories. A mere collector of facts is far from being a real man of Science but there is room for real division of labour in contributing to the growth of Science and this implies that the work of various grades of investigators can be of value in the construction of the edifice. Among these grades those workers who undertake the arduous duty under suitable direction of collecting precise facts in some given experimental or observational domain have an honourable place. But in the trenchant words1 of Poincaré: “Science is built up of facts as a house is built up of stones; but an accumulation of facts is no more a science than a heap of stones is a house.” The really important general theories are always the work of men endowed with very exceptional powers of imagination insight and generalization which enable them to seize instinctively upon relations of similarity between things that have little or no superficial resemblance with one another. These great pioneers have formed but a small band in the history of Science.
Even after medieval conceptions of Science had waned the Rationalistic school which preceded Kant and the critical conceptions he initiated attempted to construct nature out of pure Thought. The critical movement led by Hume and Kant established the impotence of Thought alone devoid of the essential foundation of actual physical experience to formulate any genuinely scientific view of the world of physical phenomena.
In what I have said hitherto about the functions of Natural Science the conceptual description of the physical world alone has been referred to but it is clear that no treatment of the general aspects of the subject Would be adequate without some reference to the psychical order of things. That the physical and psychical domains have in appearance at least very close relations with one another is obvious to the most casual observation. Prima facie they are not independent of one another but closely interconnected.
Modern Physiology on the one side and Modern Psychology on the other side have confirmed this fact of observation in minute detail. There is evidence of overwhelming strength that psychical events or processes in the mind are accompanied by physical events or processes in the bodily organism; that at least some kinds of physical events or processes in the body are accompanied by psychical happenings in the mind is also a fact not open to doubt. The question of the nature of the relations between such physical and psychical processes between body and mind or of how such relations are to be represented is one of the most difficult and intractable problems of Philosophy. An important and influential school of Physiologists has advocated the view that physical processes in the brain and nervous system of the bodily organism form a closed sequence entirely independent of the psychical processes or happenings which accompany these physical processes; that despite appearances the sequences of events in the bodily organism are entirely unaffected by concomitant psychical sequences. In accordance with this view if we possessed a sufficiently advanced knowledge of Physiology a complete account involving only the categories of Physics and Chemistry could be given of all the actions of a man. of all his responses to external stimuli without taking into account his consciousness or will or any of the motives to which he himself attributes his actions.
His cognition feelings and conation are in this theory regarded as belonging to a domain which has no influence upon the world of physical phenomena including all the physical happenings in his own body; the former are regarded as epiphenomena or Begleiterscheinungen which accompany but have no influence upon the latter. The man is a conscious automaton a machine endowed with consciousness but not with the power to influence his own actions; although he is under the delusion that he has this power. The closeness of the correspondence between the sequences of events in the two domains has been formulated in the theory known as Psycho-physical Parallelism. Whatever be the value of this theory it certainly makes colossal demands upon our powers of imagination when we attempt to represent to ourselves how it works and what it implies in an individual case. On the 7th of May 1915 multitudes of people read telegrams conveying the news that the Lusitania had been torpedoed. The reading of the telegram was followed in a vast number of cases by bodily feelings indicating very marked disturbances in the nervous system. That these nervous tremors were due to a conscious apprehension of the terrible meaning of the news is the ordinary view of common sense. But a believer in the theory of psycho-physical parallelism is bound to assert that the meaning of the telegram only interpretable in psychical terms had nothing whatever to do with these nervous disturbances. The image of the print of the telegram on the retina of the eye the subsequent neural currents to the brain certain changes in the smallest part of the grey matter of the brain and neural currents from the brain to other parts of the organism formed a sequence which could theoretically be accounted for as a sequence of purely physical phenomena; the only relevant factors in the determination of this sequence were the image on the retina and the detailed physical constitution and condition of the individual organism. A slight change in the words of the telegram such as for example the insertion of the word “not” with the corresponding slight change in the image on the retina might have led to the absence of all the nervous disturbances in the body of the reader of the telegram. A similar absence of nervous disturbance would have been observed in the case of a person who did not understand the language in which the telegram was written.
The change of meaning of the telegram would have been of vital importance but believers in the theory of psycho-physical parallelism must assert that the slight change in the image on the retina was the only factor which could account for the absence instead of the presence of what were in many cases very marked disturbances in the nervous system. This illustration has been given solely with the intention of making clear some of the implications of the theory of psycho-physical parallelism. It is not put forward as a refutation of the theory; a much more complete discussion of the theory would be required for such a purpose and it is no part of my programme to put forward even a tentative solution of the problem to which the theory is related.
Another attempt to overcome the difficulties connected with the relations between physical and psychical phenomena in the living organism is embodied in the theory of interaction. In accordance with this theory neither the physical nor the psychical phenomena form a closed system but each group is affected by action from the other. So long as the two domains are regarded as completely disparate the difficulty of this theory arises from inability to represent the precise nature of such interaction or to discover the exact points in a physical sequence at which the effect of the psychical domain makes itself apparent as a disturbing factor introduced extraneously into the physical sequence.
It has sometimes been assumed that regarding the physical phenomena in the living organism as forming a dynamical system the action of the psychical side of the organism is of such a character that no breach of the conservation of energy in the dynamical system takes place. If it could be established that the action of the psychical side is such that it does no mechanical work in the dynamical system an interesting fact would have been discovered but the main principle would not be thereby affected that the dynamical system ceases to be an independent system the working of which can be described simply in accordance with the laws of Dynamics. It must be remembered that the principle of conservation of energy itself leads only to one of the equations which describe the motions of the parts of a material system possessing more than one degree of freedom. The effect produced by the psychical factor on the physical system could only be represented by an action to which there corresponds no reaction on the physical side. Accordingly the physical part of the organism could only be treated as a dynamical system if we had a knowledge of the character and numerical measures of the mechanical forces acting on the physical system from without by which the action of the psychical part of the organism on the physical system could be represented.
There is much force in the contention that the whole problem of the nature of the relations between the physical and psychical domains in the living organism is essentially insoluble because the problem is a purely artificial one having arisen from the original assumption made that the physical and psychical sides are disparate without an underlying unity. If it be held that we have in treating body and mind as belonging to separate domains set up a distinction which does not correspond to any really fundamental difference this may be held to account for our inability to formulate any satisfactory and coherent theory of the relations between the two artificially separated domains. Having made an arbitrary separation into two domains supposed to be disparate in their natures we are unable without contradiction to undo our work by recombining them into one system. In default of a purely monistic Science which should take up into itself all the phenomena which we now call physical and also those which we call psychical and which should be sufficiently advanced as to absorb and unify all our present knowledge on both sides we are compelled to retain the methodological procedure of considering physical and psychical phenomena separately. Unless we are prepared to adopt as a general principle of unlimited scope the theory of psycho-physical parallelism that the physical domain is entirely independent of the psychical we are compelled to restrict ourselves on the side of Natural Science to the procedure of tracing out such sequences of physical events as we find by actual experience to be capable at least to a practically sufficient degree of approximation of conceptual description in which the concepts arise from the physical domain alone. That this is possible even in the biological departments of Science to some very considerable extent the limits of which we do not know has been abundantly established by modern Physiology. Nevertheless it is impossible to regard the physical side of the living organism as a completely independent system all the happenings in which are capable of being described conceptually in accordance with the canons of purely physical Science and to an extent without theoretical limits unless we are prepared to adopt the unproved assumption that the physical organism is independent of the psychical side of the living being. The amount of partial independence is just what experience shows it to be but no number of successes in subsuming particular chains of processes which occur in the living organism under conceptual schemes such as we employ in Natural Science will warrant us in asserting the principle of the absolute independence of the physical side of the organism to be more than a surmise.
We are thus led to what must be regarded as a limitation upon the claims of Natural Science in the sense of the term which has been here adopted to the power of theoretically extending itself so as to become a complete Philosophy of Physical Nature independent of all psychical factors. Physiology is completely justified in assuming this independence as a methodological principle and experience alone can decide how far it will be able to extend its present far-reaching results in accordance with that principle. But the existence of Psychology the Science of the normal individual mind with the borderland domain of Physiological Psychology indicates that the possibility of representing the phenomena of the bodily organism by means of conceptual schemes of the kind which we class as belonging to Natural Science may have limits which cannot be passed however far Physiological investigation may at present be from having reached those limits.
Hitherto I have spoken only of Natural Science as the Science of physical phenomena but in the wider sense the term Science is employed in relation to the study not only of physical but also of psychical phenomena; and it is also used in the case of the study of complexes involving both physical and psychical phenomena. The question arises how far the methods of procedure which Natural Science adopts are applicable where psychical phenomena are involved. The Science of Psychology occupies itself with the conceptual description of sequences of psychical happenings in the normal human mind; the idiosyncrasies of particular minds being disregarded just as in the conceptual schemes of Natural Science the irrelevant peculiarities of individual physical objects are disregarded. There are however important respects in which Psychology differs from a department of Natural Science. The only mental phenomena of which an observer can directly take cognisance are those which occur in his own mind: what occurs in the minds of other persons he can only ascertain indirectly through physical manifestations. Thus introspection and inference from physical events assumed to afford sufficient indications of corresponding psychical events are the two sources of the facts with which the Psychologist has to deal in ascertaining laws and in building up his conceptual schemes of representation of psychical sequences. Moreover psychical processes and states are not of a quantitative character such as are accessible to the methods of measurement employed in the physical sciences. These measurements of quantity are dependent upon that property of extension which has no direct correlative in the psychical domain. Psychical states and processes may lend themselves to notions of magnitude but not to that of extensive magnitude; an intensive magnitude not being made up of units and therefore not being capable of numerical representation as is an extensive magnitude. Such measurements as are made by experimental Psychologists are measurements of the physical concomitants of psychical phenomena and not of those phenomena themselves. It has moreover been pointed out by Professor James Ward that Psychology cannot be defined by reference to a special subject matter as in a department of physical science since it deals in some sense with the whole of experience. Not only has Psychology to concern itself with a more complex subject matter than a department of Natural Science but the position of the observer or percipient relative to the phenomena is less simple in the former case than in the latter; and it is thus difficult to assert that psychical events and processes and phenomena or factors in the subject-object relation in quite the same sense in Psychology as in the Natural Sciences. The Sciences of the Politico-social group so far as they rise above the more superficial classification of special kinds of phenomena and their statistical setting are all dependent on psychological knowledge and share in the peculiarities I have mentioned which differentiate Psychology from the Natural Sciences.
The methods of Psychology and of the Sciences of the Politico-social group must be similar in character and spirit to those pursued in the case of the Natural Sciences so far as the nature of their respective subject matter allows; and there exist in all the former Sciences tracts in which the results and methods of Natural Science are directly applicable and may render great services. Some departments of Biological Science especially evolutionary Biology and the theory of Heredity have made use of psychical categories in such a manner that they cannot unless such psychical elements can be eliminated be regarded as falling wholly under the denomination Natural Science in the restricted sense in which I have employed the term. For example Darwin in his Origin of Species occupies himself to a considerable extent with mental factors such as sexual selection as contributing the causes of the natural selection which determines the evolution of species. Purposiveness and all teleological conceptions are foreign to Natural Science; and all departments of Biology so far as they make use of such conceptions must be considered as mixed Sciences in the sense that they make use of concepts which represent not only physical percepts but also psychical elements.
The psycho-physical parallelist may maintain that the psychical elements in Biology can be ultimately eliminated and the whole be reduced to a purely physical Science; but such assertion is of a highly speculative and contentious character. In view of what has been said earlier in this lecture it cannot be assumed as established that such complete elimination of the psychical factor is possible even in theory.
From the book: