"Our principle states that if this implies that, and this is true, then that is true. In other words, 'anything implied by a proposition is true', or 'whatever follows from a true proposition is true'. This principle is really involved -- at least, concrete instances of it are involved -- in all demonstrations. Whenever one thing which we believe is used to prove something else, which we consequently believe, this principle is relevant. If any one asks: 'Why should I accept the results of valid arguments based on true premisses?' we can only answer by appealing to our principle. In fact, the truth of the principle is impossible to doubt, and its obviousness is so great that at first sight it seems almost trivial. Such principles, however, are not trivial to the philosopher, for they show that we may have indubitable knowledge which is in no way derived from objects of sense."
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The Problems of Philosophy
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