I could tell you volumes about Kant if I hadn't left all my Higher Psychology jotters at home. IIRC, he thought that both rational and empirical theory was doing it wrong, and were attempting to perceive the world backwards. Here's his page on the Internet Encyclopedia of Philosophy, which I recall referring to extensively when writing essays about him.
http://www.iep.utm.edu/kantmeta/Descartes, as I recall, was one of the first philosophical writers to separate God from philosophy. His work involved rejecting anything that could not be proven 100%, resulting in
cogito ergo sum. His philosophy was mainly rationalist, I believe, emphasizing deduction over perception.
Never heard of Piaget, I'm afraid.