There are five faculties or modes which the soul uses to arrive at truth:
- Pure science or knowledge, which belongs to the scientific faculty.
- Art or applied science, which belongs to the deliberative (calculative) faculty.
- Practical wisdom or prudence, which belongs to the deliberative faculty.
- Intuitive wisdom or intelligence, which belongs to the scientific faculty.
- Theoretical wisdom, which belongs to the scientific faculty.
In their appropriate spheres these five faculties are all, by definition, infallible. Conviction and opinion have been omitted from the list because they can both result in false conclusions.
Pure science is concerned with things that are necessary and eternal (facts which cannot be other than they are, e.g., the laws of physical science). Science pertains to knowledge that can be taught, either by induction (which starts from particulars) or syllogism (deduction, which starts from universals). In this sense scientific knowledge is a capacity for demonstration and proof. The syllogistic (deductive) process is the true scientific method, although the first principles from which syllogism proceeds are provided by induction. (For an explanation of syllogistic reasoning, see the section on Aristotle's logical method, page 9.)