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330 the operations of thoughts and to the very existence of intelligence. No thinking can possibly go on without them; to suppose that it can is to suppose an absurdity and a contradiction. The necessity that characterises ideas is of the highest and most stringent order. And, accordingly, the theory which expounds them must be accepted, not as a doctrine which may possibly be erroneous, but as a system of truth which cannot possibly be mistaken. In its expression, this theory may probably be defective; indeed it may be impossible to express it in terms which are not more or less imperfect, but in itself, and substantially, it cannot be fallacious.

20. The necessity, the necessary truth, which is the main characteristic of ideas, and which marks this theory, will become conspicuous if we make the attempt to carry on thinking without the instrumentality of ideas, that is, of universals. This attempt will show how essential ideas are to the operations of thought, and how impossible it is for thought to be performed without them. Let us, then, make the attempt; let us try whether we can think without anything more than sensation coming into play. I have a sensation of light, and a bright object, say a gas lamp, is before my eyes. Now, so long as I am merely in a state of feeling, I am tied down to this particular sensation; my sensation does not overstep one hair's-breadth the sensation which I experience. The sensation is exactly that sensation, and nothing