V. ' SENSIBILIA ' AND ' THINGS '

But if " sensibilia " are to be recognised as the ultimate constituents of the physical world, a long and difficult journey is to be performed before we can arrive either at
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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 shiny. It is said, not wholly without plausibility, that these different shapes and different colours cannot co-exist simul- taneously in the same place, and cannot therefore both be constituents of the physical world. This argument I must confess appeared to me until recently to be irre- futable. The contrary opinion has, however, been ably maintained by Dr. T. P. Nunn in an article entitled : ' Are Secondary Qualities Independent of Perception ? "l 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 space 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 un- deserved 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 spaces, although, as we shall see, it is possible, with certain limitations, to establish a correlation between them.
1 Proc. Arist. Soc.. 1909-1910, pp. 191-218.
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What we call the different appearances of the same thing to different observers are each in a space 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 class 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 con- tinuity 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 appear- ances 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, itill it is not unnatural to suppose that, from the position
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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 par- tiality, be identified with any single one of its appear- ances, it came to be thought of as something distinct from all of them and underlying them. But by the prin- ciple of Occam's razor, if the class 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 class of its appearances. It is not necessary to deny a substance or substratum underly- ing these appearances ; it is merely expedient to abstain from asserting this unnecessary entity. Our procedure here is precisely analogous to that which has swept away from the philosophy of mathematics the useless menagerie of metaphysical monsters with which it used to be in- fested.