Part 8 (1/2)
III. SENSIBILIA
I shall give the name _sensibilia_ to those objects which have the same metaphysical and physical status as sense-data, without necessarily being data to any mind. Thus the relation of a _sensibile_ to a sense-datum is like that of a man to a husband: a man becomes a husband by entering into the relation of marriage, and similarly a _sensibile_ becomes a sense-datum by entering into the relation of acquaintance. It is important to have both terms; for we wish to discuss whether an object which is at one time a sense-datum can still exist at a time when it is not a sense-datum. We cannot ask ”Can sense-data exist without being given?” for that is like asking ”Can husbands exist without being married?” We must ask ”Can _sensibilia_ exist without being given?” and also ”Can a particular _sensibile_ be at one time a sense-datum, and at another not?” Unless we have the word _sensibile_ as well as the word ”sense-datum,” such questions are apt to entangle us in trivial logical puzzles.
It will be seen that all sense-data are _sensibilia_. It is a metaphysical question whether all _sensibilia_ are sense-data, and an epistemological question whether there exist means of inferring _sensibilia_ which are not data from those that are.
A few preliminary remarks, to be amplified as we proceed, will serve to elucidate the use which I propose to make of _sensibilia_.
I regard sense-data as not mental, and as being, in fact, part of the actual subject-matter of physics. There are arguments, shortly to be examined, for their subjectivity, but these arguments seem to me only to prove _physiological_ subjectivity, i.e. causal dependence on the sense-organs, nerves, and brain. The appearance which a thing presents to us is causally dependent upon these, in exactly the same way as it is dependent upon intervening fog or smoke or coloured gla.s.s. Both dependences are contained in the statement that the appearance which a piece of matter presents when viewed from a given place is a function not only of the piece of matter, but also of the intervening medium.
(The terms used in this statement--”matter,” ”view from a given place,” ”appearance,” ”intervening medium”--will all be defined in the course of the present paper.) We have not the means of ascertaining how things appear from places not surrounded by brain and nerves and sense-organs, because we cannot leave the body; but continuity makes it not unreasonable to suppose that they present _some_ appearance at such places. Any such appearance would be included among _sensibilia_.
If--_per impossibile_--there were a complete human body with no mind inside it, all those _sensibilia_ would exist, in relation to that body, which would be sense-data if there were a mind in the body. What the mind adds to _sensibilia_, in fact, is _merely_ awareness: everything else is physical or physiological.
IV. SENSE-DATA ARE PHYSICAL
Before discussing this question it will be well to define the sense in which the terms ”mental” and ”physical” are to be used. The word ”physical,” in all preliminary discussions, is to be understood as meaning ”what is dealt with by physics.” Physics, it is plain, tells us something about some of the const.i.tuents of the actual world; what these const.i.tuents are may be doubtful, but it is they that are to be called physical, whatever their nature may prove to be.
The definition of the term ”mental” is more difficult, and can only be satisfactorily given after many difficult controversies have been discussed and decided. For present purposes therefore I must content myself with a.s.suming a dogmatic answer to these controversies. I shall call a particular ”mental” when it is aware of something, and I shall call a fact ”mental” when it contains a mental particular as a const.i.tuent.
It will be seen that the mental and the physical are not necessarily mutually exclusive, although I know of no reason to suppose that they overlap.
The doubt as to the correctness of our definition of the ”mental” is of little importance in our present discussion. For what I am concerned to maintain is that sense-data are physical, and this being granted it is a matter of indifference in our present inquiry whether or not they are also mental. Although I do not hold, with Mach and James and the ”new realists,” that the difference between the mental and the physical is _merely_ one of arrangement, yet what I have to say in the present paper is compatible with their doctrine and might have been reached from their standpoint.
In discussions on sense-data, two questions are commonly confused, namely:
(1) Do sensible objects persist when we are not sensible of them? in other words, do _sensibilia_ which are data at a certain time sometimes continue to exist at times when they are not data? And (2) are sense-data mental or physical?
I propose to a.s.sert that sense-data are physical, while yet maintaining that they probably never persist unchanged after ceasing to be data. The view that they do not persist is often thought, quite erroneously in my opinion, to imply that they are mental; and this has, I believe, been a potent source of confusion in regard to our present problem. If there were, as some have held, a _logical impossibility_ in sense-data persisting after ceasing to be data, that certainly would tend to show that they were mental; but if, as I contend, their non-persistence is merely a probable inference from empirically ascertained causal laws, then it carries no such implication with it, and we are quite free to treat them as part of the subject-matter of physics.
Logically a sense-datum is an object, a particular of which the subject is aware. It does not contain the subject as a part, as for example beliefs and volitions do. The existence of the sense-datum is therefore not logically dependent upon that of the subject; for the only way, so far as I know, in which the existence of _A_ can be _logically_ dependent upon the existence of _B_ is when _B_ is part of _A_. There is therefore no _a priori_ reason why a particular which is a sense-datum should not persist after it has ceased to be a datum, nor why other similar particulars should not exist without ever being data. The view that sense-data are mental is derived, no doubt, in part from their physiological subjectivity, but in part also from a failure to distinguish between sense-data and ”sensations.” By a sensation I mean the fact consisting in the subject's awareness of the sense-datum. Thus a sensation is a complex of which the subject is a const.i.tuent and which therefore is mental. The sense-datum, on the other hand, stands over against the subject as that external object of which in sensation the subject is aware. It is true that the sense-datum is in many cases in the subject's body, but the subject's body is as distinct from the subject as tables and chairs are, and is in fact merely a part of the material world. So soon, therefore, as sense-data are clearly distinguished from sensations, and as their subjectivity is recognised to be physiological not psychical, the chief obstacles in the way of regarding them as physical are removed.
V. ”SENSIBILIA” AND ”THINGS”
But if ”sensibilia” are to be recognised as the ultimate const.i.tuents of the physical world, a long and difficult journey is to be performed before we can arrive either at the ”thing” of common sense or at the ”matter” of physics. The supposed impossibility of combining the different sense-data which are regarded as appearances of the same ”thing” to different people has made it seem as though these ”sensibilia” must be regarded as mere subjective phantasms. A given table will present to one man a rectangular appearance, while to another it appears to have two acute angles and two obtuse angles; to one man it appears brown, while to another, towards whom it reflects the light, it appears white and s.h.i.+ny. It is said, not wholly without plausibility, that these different shapes and different colours cannot co-exist simultaneously in the same place, and cannot therefore both be const.i.tuents of the physical world. This argument I must confess appeared to me until recently to be irrefutable. The contrary opinion has, however, been ably maintained by Dr. T.P. Nunn in an article ent.i.tled: ”Are Secondary Qualities Independent of Perception?”[29] The supposed impossibility derives its apparent force from the phrase: ”_in the same place_,” and it is precisely in this phrase that its weakness lies. The conception of s.p.a.ce is too often treated in philosophy--even by those who on reflection would not defend such treatment--as though it were as given, simple, and unambiguous as Kant, in his psychological innocence, supposed. It is the unperceived ambiguity of the word ”place” which, as we shall shortly see, has caused the difficulties to realists and given an undeserved advantage to their opponents. Two ”places” of different kinds are involved in every sense-datum, namely the place _at_ which it appears and the place _from_ which it appears. These belong to different s.p.a.ces, although, as we shall see, it is possible, with certain limitations, to establish a correlation between them. What we call the different appearances of the same thing to different observers are each in a s.p.a.ce private to the observer concerned. No place in the private world of one observer is identical with a place in the private world of another observer. There is therefore no question of combining the different appearances in the one place; and the fact that they cannot all exist in one place affords accordingly no ground whatever for questioning their physical reality. The ”thing” of common sense may in fact be identified with the whole cla.s.s of its appearances--where, however, we must include among appearances not only those which are actual sense-data, but also those ”sensibilia,” if any, which, on grounds of continuity and resemblance, are to be regarded as belonging to the same system of appearances, although there happen to be no observers to whom they are data.
An example may make this clearer. Suppose there are a number of people in a room, all seeing, as they say, the same tables and chairs, walls and pictures. No two of these people have exactly the same sense-data, yet there is sufficient similarity among their data to enable them to group together certain of these data as appearances of one ”thing” to the several spectators, and others as appearances of another ”thing.”
Besides the appearances which a given thing in the room presents to the actual spectators, there are, we may suppose, other appearances which it would present to other possible spectators. If a man were to sit down between two others, the appearance which the room would present to him would be intermediate between the appearances which it presents to the two others: and although this appearance would not exist as it is without the sense organs, nerves and brain, of the newly arrived spectator, still it is not unnatural to suppose that, from the position which he now occupies, _some_ appearance of the room existed before his arrival. This supposition, however, need merely be noticed and not insisted upon.
Since the ”thing” cannot, without indefensible partiality, be identified with any single one of its appearances, it came to be thought of as something distinct from all of them and underlying them.
But by the principle of Occam's razor, if the cla.s.s of appearances will fulfil the purposes for the sake of which the thing was invented by the prehistoric metaphysicians to whom common sense is due, economy demands that we should identify the thing with the cla.s.s of its appearances. It is not necessary to _deny_ a substance or substratum underlying these appearances; it is merely expedient to abstain from a.s.serting this unnecessary ent.i.ty. Our procedure here is precisely a.n.a.logous to that which has swept away from the philosophy of mathematics the useless menagerie of metaphysical monsters with which it used to be infested.
VI. CONSTRUCTIONS VERSUS INFERENCES
Before proceeding to a.n.a.lyse and explain the ambiguities of the word ”place,” a few general remarks on method are desirable. The supreme maxim in scientific philosophising is this:
_Wherever possible, logical constructions are to be subst.i.tuted for inferred ent.i.ties._
Some examples of the subst.i.tution of construction for inference in the realm of mathematical philosophy may serve to elucidate the uses of this maxim. Take first the case of irrationals. In old days, irrationals were inferred as the supposed limits of series of rationals which had no rational limit; but the objection to this procedure was that it left the existence of irrationals merely optative, and for this reason the stricter methods of the present day no longer tolerate such a definition. We now define an irrational number as a certain cla.s.s of ratios, thus constructing it logically by means of ratios, instead of arriving at it by a doubtful inference from them. Take again the case of cardinal numbers. Two equally numerous collections appear to have something in common: this something is supposed to be their cardinal number. But so long as the cardinal number is inferred from the collections, not constructed in terms of them, its existence must remain in doubt, unless in virtue of a metaphysical postulate _ad hoc_. By defining the cardinal number of a given collection as the cla.s.s of all equally numerous collections, we avoid the necessity of this metaphysical postulate, and thereby remove a needless element of doubt from the philosophy of arithmetic.
A similar method, as I have shown elsewhere, can be applied to cla.s.ses themselves, which need not be supposed to have any metaphysical reality, but can be regarded as symbolically constructed fictions.