_S is P_: that is a model logical judgment; there can be no question of believing it; but it is logically valid if _M is P_ and _S is M_.

When, again, in Logic, one deals with belief, it depends upon evidence; whereas, in Psychology belief is shown to depend upon causes which may have evidentiary value or may not; for Psychology explains quite impartially the growth of scientific insight and the growth of prejudice.
(c) Mill, Bain, and Venn are the chief Materialist logicians; and to guard against the error of confounding Materialism in Logic with the ontological doctrine that nothing exists but Matter, it may suffice to remember that in Metaphysics all these philosophers are Idealists.
Materialism in Logic consists in regarding propositions as affirming or denying relations (_cf._ Sec.

5) between matters-of-fact in the widest sense; not only physical facts, but ideas, social and moral relations; it consists, in short, in attending to the meaning of propositions.

It treats the first principles of Contradiction and Causation as true of things so far as they are known to us, and not merely as conditions or tendencies of thought; and it takes these principles as conditions of right thinking, because they seem to hold good of Nature and human life.
To these differences of opinion it will be necessary to recur in the next chapter (Sec.

4); but here I may observe that it is easy to exaggerate their importance in Logic.