51. What Kind of Necessity Attaches to the Principles of the Syllogism? -- The law which serves as a fulcrum for reasoning is sometimes metaphysical, or absolute (see example given under 50); sometimes physical, or natural, and therefore dependent on conditions to be determined by experience (as: Water attains its maximum density at 40 centigrade).

In the former case the predicate in the conclusion expresses the essence, total or partial, of the middle term, or a property which is a corollary of that essence, and the necessity of applying this predicate to the subject of the conclusion is absolute.

In the latter case the quality is attributed to the middle term in virtue of a law established by experience, and the attribution of predicate to subject in the conclusion is hypothetically necessary. These laws of experience are established by induction, as will he seen later.