The principal tenets of the Aristotelian natural philosophy were: the doctrine of form and matter, the four causes, the rigid separation of the world into opposed spheres, and the finite nature of the universe. During the Renaissance, these precepts were both defended and revised by Aristotelian professors, or challenged by others who sought to dismantle traditional philosophy. While these new philosophers could rely on new evidence, methods, and observations to define the nature of the universe, in other cases the rejection of Aristotelian doctrines and their substitution with new paradigms was mainly based on speculative arguments. 