A System of Logic

MS 169: Winter-Spring 1870

Chapter I. Syllogism

§1. Why we begin with syllogism

The cause of the existence of logic has as a historical fact been a desire to have a science for testing inferences. That logic serves other ends and is susceptible of a higher definition I admit; but that cannot be understood at the beginning.

Before Aristotle, logic consisted of an inductive method of reaching definitions and a large quantity of sophisms. The former was a mode of drawing inferences so that they should be certain. And what was the interest of these sophisms? To prove, perhaps, that things were self-contradictory. But so far they created no logical knowledge; only when the interest in them came to be the trying of the sufficiency and validity of adopted forms of thought did it at once become necessary to have a science of the syllogism. I think that it must have been that the consideration of such sophisms did much to creating Aristotle's syllogistic theory. In so far as this is a true account of the origin of logic, it originated in a desire to form a theory of inference.

But, as Duns Scotus shows, 1 whatever logic treats of besides inferences, that is to say, propositions and terms, are only parts of syllogisms and these are only treated in logic in their reference to syllogism. Why is the grammatical treatment of propositions so different from that of logic? Merely because logic only considers those differences between propositions upon which differences between syllogisms depend. Logic says nothing about interrogative, imperative, and optative sentences which are of fundamental importance in the general theory of propositions; but on the other hand the distinctions of affirmative and negative, universal and particular which are hardly worth mentioning in grammar are always allowed to be of the highest importance in logic. The reason plainly is that interrogations, commands, and wishes can form no part of a syllogism, while the difference between a valid and an invalid syllogism will often depend on the difference between a universal and a particular or on that between an affirmative and a negative proposition. The same thing may be said of terms. The principles of extension and comprehension, distinctness and confusion, which logic considers, are of importance relatively to the theory of inferences, while the distinctions of noun and adjective, compound names and simple names, singulars and plurals, together with the innumerable distinctions of verbs which it omits have no apparent bearing upon inference.

Having this conviction, I believe it to be altogether wrong to treat propositions and terms before syllogism; for since no distinctions of propositions and terms are to be introduced which the theory of syllogism does not require, none ought to be introduced before the theory of syllogism has shown the necessity of it.

[See Note A. On the Definition of Logic.]

§2. On the Essential Characters of an Argument

During the thirteenth century, a form of argument different from the syllogism came into use, which was called the consequentia and which deserves consideration. Here is an actual example of such an argument which I copy from Duns Scotus and which I will make the text of an explanation.

It is an argument to show that the subject of Aristotle's book of the categories is not the ten categories, founded on the assumption that it is a logical work. I abridge the argument a little.


De istis est scientia realis; non ergo Logica. Consequentia patet per Aristotelem 3. de Anima, cont. 38. Scientiae secantur, ut res; sed Logica est diversa ab omni scientia reali; ergo subiectum eius a subiecto cuiuslibet scientiae realis est diversum. Probatio antecedentis; quia de istis determinat Metaphysicus, ut patet 5 Metaph. cont. 14. et inde, et 7. Metaphysicae, cont. 1. et inde.


This I should translate as follows. A real science treats of the ten categories; logic, therefore, does not. The consequence (the validity of bond between antecedent and consequent) appears from what Aristotle says in the third book De Anima contextus 38. "The divisions of the sciences follow the divisions of the objects to which they relate." Now logic is other than any real science and consequently its subject is other than that of any real science. Proof of the antecedent: metaphysics treats of the ten categories, as appears from two passages in Aristotle.

It will be perceived that this form of setting forth an argument supposes that an argument is composed of an antecedent or premiss and a consequent (consequens) or conclusion; while the validity of the argument depends on the truth of a general principle called the consequence (consequentia).

1. Super Universalia, quaestio 3. Scotus maintains that the object of logic is the syllogism.