Leibniz provides us is merely a more precise determination of the Cartesian criterion of all knowledge, namely: clara et distinctio perceptio.

In Descartes, perceptio means the same as the German Wahrnehmungin the sense of the animadvertere, comprehendere, or grasping the thing itself. Descartes further distinguishes between two actiones intellectus [acts of intellect], two different ways our knowing can comport itself: intuitus and deductio, intuition and deduction. He says that intuition is more certain than deduction because it is more simple [121] and because it does not have the thing merely indirectly, by way of the individual steps of the proof. Rather, it is with the thing itself. Characteristic of intuition is evidentia praesens [present evidence]. In intuition the thing itself is seen, and at the same time one’s seeing of the thing is itself co-seen and co-known: This is the peculiar self-certitude that intuition has and that Husserl formulated in his principle of all principles. And this Cartesian understanding of the concept of knowledge (which I will not go into further) goes back in turn to Scholasticism. I want you to see that this idea—that knowledge properly speaking is intuition—has an even greater importance than came out with Kant, and to do so I have to talk about it in some other contexts.

The notion that knowledge is intuition or direct seeing is found in the Middle Ages, not only in the context of what we call mystical, philosophical, or theological speculation, but precisely in those thinkers who understood problems in an essentially Aristotelian way. These gave great importance to argumentation and to indirect, syllogistic proof—in Descartes’s terminology, to deduction. For a thinker like Thomas Aquinas, the primacy of intuition shows up precisely in that context, and in fact intuition is not only the authentic and highest comportment within knowing but is also the highest possible way of being human, since in intuition, to the degree the human truly and properly intuits, he or she is in the presence of the highest entity—the one that most properly is, namely, God. God is the ens perfectissimum, the most-perfect entity; and insofar as God is intuited, this intuition of God is the highest kind of human being, one that humans do not have on earth but will attain only in heavenly bliss. Which means that the determination of beatitudo, happiness, is given by intuition. And why is this intuition—this pure visio Dei or pure intuition of God, this unadulterated having-God-present—why is it the highest kind of being that humans can have? Answer: Because this intuitional comportment [122] no longer looks beyond itself but is fulfilled in itself, whereas by contrast the second basic faculty of human beings as rational beings, the will, precisely as willing something, is by its very nature still unfulfilled. Willing as such is still directed toward something that it does not yet possess and that it is not yet. But in the intuition of the being itself—namely, God—there is complete fulfillment.