How, then, can we frame principles of mediate reasoning, about such things?  So far as we consider them as Kinds, it is enough to say:  Whatever can be identified as a specimen of a known substance or Kind has the properties of that Kind.  So far as we consider them as in the relation of causation, we may say:  Whatever relation of events can be identified with the relation of cause and effect is constant.  And these principles may be generalised thus:  Whatever is constantly related to a phenomenon (cause or Kind), determined by certain characters, is related in the same way to any phenomenon, that has the same characters.  Taking this as axiom of the syllogism materially treated, we see that herbivorousness, being constantly related to ruminants, is constantly related to camels; mortality to man and, therefore, to Socrates; rusting to the immersion of iron in water generally and, therefore, to this piece of iron. Nota notae, nota rei ipsius is another statement of the same principle; still another is Mill’s axiom, “Whatever has a mark has what it is a mark of.”  A mark is anything (A) that is never found without something else (B)—­a phenomenon constantly related to another phenomenon—­so that wherever A is found, B may be expected:  human nature is a mark of mortality.

Sec. 6.  The Syllogism has sometimes been discarded by those who have only seen that, as formally stated, it is either useless or fallacious:  but those who also perceive its material grounds retain and defend it.  In fact, great advantages are gained by stating an argument as a formal syllogism.  For, in the first place, we can then examine separately the three conditions on which the validity of the argument depends: 

(1) Are the Premises so connected that, if they are true, the Conclusion follows?  This depends upon the formal principles of chap. x.

(2) Is the Minor Premise true?  This question can only arise when the minor premise is a real proposition; and then it may be very difficult to answer.  Water rusts iron; but is the metal we are now dealing with a fair specimen of iron?  Few people, comparatively, know how to determine whether diamonds, or even gold or silver coins, are genuine.  That Camels are ruminants is now a verbal proposition to a Zoologist, but not to the rest of us; and to the Zoologist the ascertaining of the relation in which camels stand to such ruminants as oxen and deer, was not a matter of analysing words but of dissecting specimens.  What a long controversy as to whether the human race constitutes a Family of the Primates!  That ‘the British Empire is an empire’ affords no matter for doubt or inquiry; but how difficult to judge whether the British Empire resembles Assyria, Egypt, Rome, Spain in those characters and circumstances that caused their downfall!

(3) Is the Major Premise true?  Are all ruminants herbivorous?  If there be any exceptions to the rule, camels are likely enough to be among the exceptions.  And here the need of Inductive Logic is most conspicuous:  how can we prove our premises when they are universal propositions?  Universal propositions, however, are also involved in proving the minor premise:  to prove a thing to be iron, we must know the constant reactions of iron.