Book 6: The Intellectual Virtues.
Chapter 6: (4) Of intuitive reason as the basis of demonstrative science.
Science is a mode of judging that deals with universal and necessary truths; but truths that can be demonstrated depend upon principles, and (since science proceeds by demonstrative reasoning) every science has its principles. The principles, then, on which the truths of science depend cannot fall within the province of science, nor yet of art or prudence; for a scientific truth is one that can be demonstrated, but art and prudence have to do with that which is variable.
Nor can they fall within the province of wisdom; for it is characteristic of the wise man to have a demonstrative knowledge of certain things.
But the habits of mind or formed faculties by which we apprehend truth without any mixture of error, whether in the domain of things invariable or in the domain of things variable, are science, prudence, wisdom, and reason. If then no one of the first three (prudence, science, wisdom) can be the faculty which apprehends these principles, the only possible conclusion is that they are apprehended by reason.