History of Western Philosophy - Bertrand Russell [371]
There is a somewhat analogous fallacy as regards what is conceived. Hylas maintains that he can conceive a house which no one perceives, and which is not in any mind. Philonous retorts that whatever Hylas conceives is in his mind, so that the supposed house is, after all, mental. Hylas should have answered: 'I do not mean that I have in mind the image of a house; when I say that I can conceive a house which no one perceives, what I really mean is that I can understand the proposition "there is a house which no one perceives", or, better still, "there is a house which no one either perceives or conceives".' This proposition is composed entirely of intelligible words, and the words are correctly put together. Whether the proposition is true or false, I do not know; but I am sure that it cannot be shown to be self-contradictory. Some closely similar propositions can be proved. For instance: the number of possible multiplications of two integers is infinite, therefore there are some that have never been thought of. Berkeley's argument, if valid, would prove that this is impossible.
The fallacy involved is a very common one. We can, by means of concepts drawn from experience, construct statements about classes some or all of whose members are not experienced. Take some perfectly ordinary concept, say 'pebble'; this is an empirical concept derived from perception. But it does not follow that all pebbles are perceived, unless we include the fact of being perceived in our definition of 'pebble'. Unless we do this, the concept 'unperceived pebble' is logically unobjectionable, in spite of the fact that it is logically impossible to perceive an instance of it.
Schematically, the argument is as follows. Berkeley says: 'Sensible objects must be sensible. A is a sensible object. Therefore A must be sensible.' But if 'must' indicates logical necessity, the argument is only valid if A must be a sensible object. The argument does not prove that, from the properties of A other than its being sensible, it can be deduced that A is sensible. It does not prove, for example, that colours intrinsically indistinguishable from those that we see may not exist unseen. We may believe on physiological grounds that this does not occur, but such grounds are empirical; so far as logic is concerned, there is no reason why there should not be colours where there is no eye or brain.
I come now to Berkeley's empirical arguments. To begin with, it is a sign of weakness to combine empirical and logical arguments, for the latter, if valid, make the former superfluous.1 If I am contending that a square cannot be round, I shall not appeal to the fact that no Square in any known city is round. But as we have rejected the logical arguments, it becomes necessary to consider the empirical arguments on their merits.
The first of the empirical arguments is an odd one: That heat cannot be in the object, because 'the most vehement and intense degree of heat [is] a very great pain' and we cannot suppose 'any unperceiving thing capable of pain or pleasure'. There is an ambiguity in the word 'pain', of which Berkeley takes advantage. It may mean the painful quality of a sensation, or it may mean the sensation that has this quality. We say a broken leg is painful, without implying that the leg is in the mind; it might be, similarly, that heat causes pain, and that this is all we ought to mean when we say it is a pain. This argument,