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220 HOWARD V. KNOX : because sensations are "merely" the subjective side of cere- bral processes, therefore the world of matter, as we know it, exists only in perception. Whereas, in truth, it is only by supposing these changes, both in the brain and in the rest of the physical world, to run their course outside our con- sciousness, that we arrive at the result that sensations. are thus bound up with cerebral processes. To say, as does Professor Huxley, 1 that, scientifically, " thought is a property of matter," but that, notwithstanding this, ideal- ism may be ultimately true this is a position which, at least on the surface, seems intelligible enough. But to adopt the materialist view, and on the strength of it to airily pronounce in favour of idealism, is quite another matter. The particular fallacy animadverted on in the last para- graph is, it may be pointed out in passing, probably the best attainable specimen of an interesting species, which has never yet been given a special name. It consists in adopting premisses wbich are only admissible in virtue of a given notion or assumption, and employing them in an argument against the validity (or ' reality ') of that notion or assumption. It is to the argument what the contradictio in adjecto is to the term, or the contradic- tion in terms to the proposition. This uncouth species, which lurks chiefly in metaphysical jungles, may perhaps best be called by the name of the ignoratio principii ; since for the actual conclusion we have only to substitute its contrary in order to transform the argument into a. petitio principii. As crafty dissimulation is of the very inmost nature of this insidious monster, a schematic pre- sentment of its bare bones would be even less instructive than in the case of the remaining members of the genus.. But any one anxious to obtain other living specimens will find the Grammar of Science a rich preserve for this kind of game. We may now sum up our results in the form of the following principle the Principle of Externality which is simply the " Law of Causation " reduced to its lowest possible terms, and in connexion with which alone can the notion of externality be properly understood : In spite- of the absence of routine in the manner in which percep- tions succeed other mental phenomena and one another,, we succeed in approximately reducing the facts of experi- 1 Collected Essays, vol. i. (Method and Results). Essays III. and IV.