Kant made a bold statement in believing that both rationalists and empiricists are mistaken, which would include our old buddies Hume, Descartes, and Locke . We already know that Kant believes in synthetic a priori knowledge, which ironically would be most likely challenged by the likes of Hume, Descartes, and Locke. Kant goes on to argue that geometry, science, arithmetic, and metaphysics are all synthetic a priori. He example 7 + 5 = 12 to demonstrate why arithmetic is synthetic a priori (there are no properties of 12 in 7 or 5). The empiricists on the other hand, which include Locke and Hume, would argue that the basis of all knowledge is a posteriori, but would agree that the basis of all knowledge is also synthetic, hence they believe knowledge is only obtained by observation and that the idea that synthetic knowledge is innate or a priori. Now, the rationalists, whom Descartes happens to be, would concur that the basis of all knowledge is a priori, but would dispute that synthetic knowledge is priori.