It is easy to see that this claim itself, the principium rationis in the broadest sense, requires grounds or proof, and that this proof can obviously be given only when the essence of being as such is clarified. Inasmuch as this question is the basic question of metaphysics, the principle of reason emerges as a basic problem of metaphysics, which includes logic, the metaphysics of truth.
...we must maintain a critical caution, because the sort of generality in the vagueness is connected with the complete lack of clarity about the sense of 'principle' in this principle, something which has never, since antiquity, posed a problem. It is easy to see that the nature of principle as a problem refers to principium, to ἀρχή [arche] - thus to ground! The nature of the principle in this principle is itself to be attained only by clarifying the essence of ground as such.
Schopenhauer was the first to attempt a presentation of the principle of sufficient reason, relating all previous formulations of the problem. This remains a service...But it is totally inadequate with regard to general scientific solidity, historical presentation, as well as philosophical argument...
In section 9, "Leibniz," he says: "Leibniz is the first to have a clear conception of that distinction. In the Principiis Philosphiae he definitely distinguishes the ratio cognoscendi [reason of cognition] from the causa efficiens [efficient cause], and he presents them as two variations of the principium rationis sufficientis which he formally defines here as a main principle of all knowledge."
...Schopenhauer contributes nothing to the subject on the basis of his own philosophy, except the irritated manner. This manner took quite different forms against Schelling and Hegel and was later used by others as well; the tone cannot hide the endless superficiality with which these blustering epigones of the nineteenth century believed they had surpassed great and genuine philosophy from Plato to Hegel.
Heidegger, p.113
