13. The last two sections of this chapter direct our attention to two circumstances, which tend to prove, in a manner which we may term irresistible, the truth of the theories which they characterize:--the _Consilience of Inductions_ from different and separate classes of facts;--and the progressive _Simplification of the Theory_ as it is extended to new cases. These two Characters are, in fact, hardly different; they are exemplified by the same cases. For if these Inductions, collected from one class of facts, supply an unexpected explanation of a new class, which is the case first spoken of, there will be no need for new machinery in the hypothesis to apply it to the newly-contemplated facts; and thus, we have a case in which the system does not become {96} more complex when its application is extended to a wider field, which was the character of true theory in its second aspect. The Consiliences of our Inductions give rise to a constant Convergence of our Theory towards Simplicity and Unity. But, moreover, both these cases of the extension of the theory, without difficulty or new suppositions, to a wider range and to new classes of phenomena, may be conveniently considered in yet another point of view; namely, as successive steps by which we gradually ascend in our speculative views to a higher and higher point of generality. For when the theory, either by the concurrence of two indications, or by an extension without complication, has included a new range of phenomena, we have, in fact, a new induction of a more general kind, to which the inductions formerly obtained are subordinate, as particular cases to a general proposition. We have in such examples, in short, an instance of _successive generalization_. This is a subject of great importance, and deserving of being well illustrated; it will come under our notice in the next chapter. {{97}} CHAPTER VI. OF THE LOGIC OF INDUCTION. APHORISM XVII. _The_ Logic of Induction _consists in stating the Facts and the Inference in such a manner, that the Evidence of the Inference is manifest: just as the Logic of Deduction consists in stating the Premises and the Conclusion in such a manner that the Evidence of the Conclusion is manifest._ APHORISM XVIII. _The Logic of Deduction is exhibited by means of a certain Formula; namely, a Syllogism; and every train of deductive reasoning, to be demonstrative, must be capable of resolution into a series of such Formulæ legitimately constructed. In like manner, the Logic of Induction may be exhibited by means of certain_ Formulæ; _and every train of inductive inference to be sound, must be capable of resolution into a scheme of such Formulæ, legitimately constructed._ APHORISM XIX. _The_ inductive act of thought _by which several Facts are colligated into one Proposition, may be expressed by saying:_ The several Facts are exactly expressed as one Fact, if, and only if, we adopt the Conceptions and the Assertion _of the Proposition._