28. In men as in brutes, the consecutiveness of their perceptions is due to the principle of memory—like empirics in medicine, who have only practice without theory. And we are mere empirics in three-fourths of our acts. For example, when we expect that the sun will rise to-morrow, we judge so empirically, because it has always risen hitherto. Only the astronomer judges by an act of reason. 29. But the cognition of necessary and eternal truths is that which distinguishes us from mere animals. It is this which gives us Reason and Science, and raises us to the knowledge of ourselves and of God; and it is this in us which we call a reasonable soul or spirit. 30. It is also by the cognition of necessary truths, and by their abstractions, that we rise to acts of reflection, which give us the idea of that which calls itself “I,” and which lead us to consider that this or that is in us. And thus, while thinking of ourselves, we think of Being, of substance, simple or compound, of the immaterial, and of God himself. We conceive that that which in us is limited, is in him without limit. And these reflective acts furnish the principal objects of our reasonings. 31. Our reasonings are founded on two great principles, that of “_Contradiction_,” by virtue of which we judge that to be false which involves contradiction, and that to be true which is opposed to, or which contradicts the false. 32. And that of the “_Sufficient Reason_,” by virtue of which we judge that no fact can be real or existent, no statement true, unless there be a sufficient reason why it is thus, and not otherwise, although these reasons very often cannot be known to us. 33. There are also two sorts of truths—those of reasoning and those of fact. Truths of reasoning are necessary, and their opposite is impossible; those of fact are contingent, and their opposite is possible. When a truth is necessary, we may discover the reason of it by analysis, resolving it into simpler ideas and truths, until we arrive at those which are ultimate.[3] 34. It is thus that mathematicians by analysis reduce speculative theorems and practical canons to definitions, axioms and postulates. 35. And finally, there are simple ideas of which no definition can be given; there are also axioms and postulates,—in one word, _ultimate principles_, which cannot and need not be proved. And these are “Identical Propositions,” of which the opposite contains an express contradiction.