"If all [the mind] had was the mere capacity to receive those items of knowledge--a passive power to do so, as indeterminate as the power of wax to receive shapes or of a blank page to receive words--it would not be the source of necessary truths"
— Leibniz, Gottfried Wilhelm (1646-1716)
The mind is capable not merely of knowing them, but also of finding them within itself. If all it had was the mere capacity to receive those items of knowledge--a passive power to do so, as indeterminate as the power of wax to receive shapes or of a blank page to receive words--it would not be the source of necessary truths, as I have just shown that it is. For it cannot be denied that the senses are inadequate to show their necessity, and that therefore the mind has a disposition (as much active as passive) to draw them from its own depths; thoughthe senses are necessary to give the mind the opportunity and the attention for this, and to direct it towards certain necessary truths rather than others. So you see, sir, that these people who hold a different view, able though they are, have apparently failed to think through the implications of the distinction between necessary or eternal truths and truths of experience. I said this before, and our entire debate confirms it. The fundamental proof of necessary truths comes from the understanding alone, and other truths come from experience fo from observations of the senses. Our mind is capable of knowing truths of both sorts, but it is the source of the former; and however often one experienced instances of universal truth, one could never know inductively that it would always hold unless one knew through reason that it was necessary.
See Nouveaux Essais sur l'entendement humain in Oeuvres Philosophiques (Amsterdam and Leipzig: Jean Schreuder, 1765). <Link to Google Books>
Reading a modern translation: New Essays on Human Understanding. trans. and ed. by Peter Remnant and Jonathan Bennett. (Cambridge: Cambridge UP, 1996).