The correct answer is: A: The idea that reason is the main source and test of knowledge is at the core of rationalism. Rationalism is a theory or a methodology which considers that the criterion of the truth is not sensory but intellectual and deductive. Rationalists believe that reality has a logical structure which can be deduced by the power of man's reason. Some of the most important philosophers of rationalism in antique were: Pythagoras, Plato and Aristotle, and in the modern rationalism: Rene Descartes, Baruch Spinoza, Gottfried Leibniz and Immanuel Kant.