181. So far we have only considered Berkeley’s reduction of primary qualities, supposed to be sensible, to sensations as it affects the qualities themselves, rather than as it affects the possibility of universal judgments about them. If, indeed, as we have found, such reduction really amounts to the absolute obliteration of the qualities, no further question can remain as to the possibility of general knowledge concerning them. As Berkeley, however, did not admit the obliteration, the further question did remain for him: and the condition of his plausibly answering it was that he should recognise in the ‘idea,’ as subject of predication, that intelligible qualification by relation which he did not recognise in it simply as ‘idea,’ and which essentially differences it from feeling proper. If any particular ‘tangible extension,’ _e.g._ a right-angled triangle, is only a feeling, or in Berkeley’s own language, ‘a fleeting perishable passion’ [1] not existing at all, even as an ‘abstract idea,’ except when some one’s tactual organs are being affected in a certain way--what are we to make of such a general truth as that the square on its base is always equal to the squares on its sides? Omitting all difficulties about the convertibility of a figure with a feeling, we find two questions still remain--How such separation can be made of the figure from the other conditions of the tactual experience as that propositions should be possible which concern the figure simply; and how a single case of tactual experience--that in which the mathematician finds a feeling called a right-angled triangle followed by another which he calls equality between the squares, &c.--leads in the absence of any ‘necessary connexion’ to the expectation that the sequence will always be the same. [2] The difficulty becomes the more striking when it is remembered that though the geometrical proposition in question, according to Berkeley, concerns the tangible, the experience which suggests it is merely visual. [1] ‘Principles of Human Knowledge,’ sec. 89. [2] See above, paragraph 122. His theory of universals ...