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 some- thing about some of the constituents of the actual world ; what these constituents 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 assuming a dogmatic answer to these controversies. I sliall 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 constituent.
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It wall 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 dis- cussion. 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 com- monly confused, namely :
(i) 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 assert that sense-data are physical, while yet maintaining that they probably never persist un- changed 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, 1 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 ascer- tained 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.
152 MYSTICISM AND LOGIC
Logically a sense-datum is an object, a particular ol 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 con- stituent 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 dis- tinct from the subject as tables and chairs are, and is in fact merely a part of the material world. So soon, there- fore, as sense-data are clearly distinguished from sensa- tions, and as their subjectivity is recognised to be physio- logical not psychical, the chief obstacles in the way of regarding them as physical are removed.