The investigation which deals with the world, in its totality and in each of its individual parts, is called natural investigation, while that which regards what man possesses by virtue and will is called the science of the things that depend on the will. Since that which is natural and innate in man precedes in time that which is in man by will and choice, the first type of investigation will precede the second even if both must arrive at a certain science. Besides these two types of research is the art of logic which forms the rational part of the soul, leads it to certainty, to study and to research; logic, moreover, guides and tests the validity of the other two fields of research.