In our discussion of psychology, we identified two forms of reasoning – deduction and induction. In this lecture, we'll study the former.

The most perfect form of deductive reasoning is the syllogism, which allows us to establish the truth of a new proposition on the basis of a proposition already recognized as true. For example, if the proposition to be proven is that “Paul is mortal,” we proceed by deducing it from another proposition recognized as true – “All men are mortal” – by means of the intermediary proposition “Paul is a man.”

All men (M) are mortal (T);

Paul (t) is a man (M);

Therefore, Paul (t) is mortal (T).

The last proposition contains two terms and an attribute with an extension greater than that of the subject. For this reason, the attribute is called the major term (T). The subject, by contrast, is called the minor term (t). Finally, the first two propositions contain a common term by means of which the major and minor terms are compared. This is called the middle term.

The third proposition of a syllogism is called either the question or the conclusion, depending on whether the demonstration has already taken place or not. The first two propositions are called the premises – the one in which the major term is compared to the middle term is called the major premise, while that containing the minor term is called the minor premise.