Gottfried Wilhelm Leibniz, answered from the texts and cited to the page.
Two great principles carry everything. The principle of contradiction governs necessary truths, whose opposite implies a contradiction. The principle of sufficient reason governs the contingent: nothing is the case, no proposition true, without a reason why it is so and not otherwise.1 From sufficient reason follows the identity of indiscernibles — there cannot be two substances perfectly alike, for there would be no reason to place them differently, and to differ in no respect is to be the same.
And in the complete individual concept of any substance every predicate ever true of it is already contained, so that to be a true proposition is for the predicate to be included in the subject.23 The logic and the metaphysics are bound together; but the system grows equally from the principle of the best, which logic alone cannot supply.45
Men act like beasts insofar as the sequence of their perceptions results from the principle of memory alone; they resemble the empirical physicians who practice without theory. We are all mere Empirics in three fourths of our actions. For example, when we expect that the day will dawn tomorrow, we act like an Empiric, because until now it has always been thus. Only the astronomer judges this by reason (Prelim., sec.The Principles of Philosophy, or, the Monadology (1714), pp. 359–360
Finally, I have given a decisive reason which, in my opinion, takes the place of a demonstration; this is, that always in every affirmative proposition whether veritable, necessary or contingent, universal or singular, the concept ofthe predicate is comprised" in some sortj_n_that.of the subject. Eith_er tEe preciicate is in the subject or else I do not know what truth is.Correspondence with Arnauld, pp. 132–133
The matter can be illustrated with an appropriate example from geometry and numbers. Just as in necessary propositions, where, through a continual analysis of the predicate and the subject, a thing can at last be brought to the point where it is apparent that the concept of the predicate is in the subject, so it is in the case of numbers, where, through a continual analysis (of successive divisions) we can finally…The Shorter Leibniz Texts, p. 112
We are all mere empirics in three-fourths of our actions. For example, when we expect daylight tomorrow, we act as empiricists, because this has always happened up to the present. Only an astronomer concludes it by reason. 29. But it is the knowledge of necessary and eternal truths which distinguishes us from simple animals and gives us reason and the sciences, lifting us to the knowledge of ourselves and of God.The Monadology", 1714, p. 658
But in any case, for Leibniz all truths are ultimately reducible to identities, i.e. tautologies like 'a mortal man is a man', even if it is beyond human powers to see this reduction through to the very end in most cases.The Shorter Leibniz Texts, p. 7
Russell and Couturat made the predicate-in-subject theory the master key, reading the metaphysics as a deduction from logic — the dominant interpretation of the early twentieth century. Mercer and others answered that the system's roots are equally Neoplatonic and theological. The identity of indiscernibles and the principle of sufficient reason remain live in contemporary metaphysics; whether Leibniz's logic forces his metaphysics, or merely accompanies it, is unsettled.