Here is the text box on famous rationalists. You can probably see why I've relegated these particulars to a text box. Any suggestions welcome.
Plato (ca. 427-347): Plato famously suggested that the world around us that we experience with our senses is far less real than the world we can access with our minds (see chapter 7, What Is Reality?). The world of our senses is a world of shadows cast by the truest world, the world of ideas. The things we experience with our senses are like copies or images of the ideal patterns that we can only contemplate with our minds. Our minds know these ideas because our souls had access to them before they became imprisoned in our bodies. Learning is thus remembering this innate knowledge rather than a matter of gaining new knowledge.
René Descartes (1596-1650): Descartes is known as the founder of modern philosophy primarily for the way he drew attention to the question of certainty in what we know. He notoriously set out to doubt everything he could doubt and to accept as true only those things that he could conceive clearly and distinctly. The perceptions of his senses were the first to go, securing him his place among the rationalists. Descartes suggested that I could be dreaming what I think I am experiencing. Even in my thoughts, an evil demon could be manipulating me. What I cannot doubt, he famously suggested, is that I am thinking. “I think; therefore, I am.”
Thinking that he had established a point of certainty, he proceeded to argue that God existed, because any conception of God “clearly and distinctly” includes the idea that He is a necessary Being. And if God exists, he is not an evil demon who would try to deceive us by making us perceive the world outside our minds differently than it is. Thus I can trust my mind as it thinks about the world outside it.
Baruch Spinoza (1632-1677): Spinoza took from Descartes the idea that truth is a matter of what we can conceive of clearly and distinctly. But whereas Descartes saw knowing as a matter of our souls understanding a material world outside our minds, Spinoza believed that everything was one basic thing, God. The truths we associate with our thought and the truths we associate with the world are both truths about “God” that we can discover through reason. Experience is not necessary to learn any truth. All such truth is predetermined, and understanding not least involves coming to accept it. In this Spinoza partially resembles the Stoics, although he did not oppose emotions to the extent they did. My emotions also involve truths that I must accept.
Gottfried W. Leibniz (1646-1716): The depth of Leibniz’ rationalism did not come through clearly until his unpublished writings became better known in the 1800’s. Leibniz’ two principles of reason were already known: his “law of contradiction” and “principle of sufficient reason.” Truths of reasoning are truths we can know apart from experience. The law of contradiction says that two contradictory claims cannot both be true, and we know this must be the case even before we have any relevant experiences. Truths of reasoning are thus necessary truths.
On the other hand, principles of sufficient reason relate to truths of fact. These truths are contingent rather than necessary truths—they did not have to be true but turn out to be. As human beings, we come to know these sorts of truths through our experiences. Leibniz’ unpublished writings have shed considerable light on what he was thinking here. For Leibniz, all the things that happen to us in life are dictated by reasons we will never fully know. Of all the possible things that could be, some cannot coexist because of the law of contradiction. Those things that do exist are the greatest number of possible things that could coexist.
In the end, Leibniz followed Spinoza in believing that all truths are already determined and known by God. Experience thus does not contribute to truth, even if we as humans seem to come to know truth by this path. In theory, all truth could be known through reason alone.