A modal proposition states the mode or manner in which the predicate belongs to the subject. The signs of modal propositions are the adverbs of time, place, degree, manner. Illustrations: “James is walking rapidly.” “Honesty is always the best policy.” “Aristotle was probably the greatest thinker of ancient times.”
A pure proposition simply states that the predicate belongs, or does not belong, to the subject. Illustrations: “James is walking.” “Honesty is the best policy.” “Aristotle was the greatest thinker of ancient times.”
Some logicians refer to modal propositions as being such as indicate degrees of belief. Such words as “probably,” “certainly,” etc., would indicate their modality.
As logic has to do with the pure proposition and not the modal, the difference of opinion is of little import.
(3) Truistic Propositions.
A truistic proposition is one in which the predicate repeats the words and the meaning of the subject. Illustrations: “A man is a man,” “A beast is a beast,” “A traitor is a traitor,” “What I have done I have done.”
The truistic proposition is of little importance except in cases where the subject is used extensionally while the predicate is used intensionally. In the illustration, “A man is a man,” the subject merely stands for a member of the man family, while the predicate may indicate certain manly qualities. Against such ambiguities the logician must be on guard.
8. THE RELATION BETWEEN SUBJECT AND PREDICATE.
In Chapter 5 the extension and intension of terms was explained. The student recalls, for instance, that the term “man” may be used to denote objects, as “white man,” “black man,” “red man,” etc. In this sense the term “man” is used extensionally. But when made to stand for the attributes “rationality,” “power of speech,” etc., the term “man” is used intensionally.
In considering the relation between subject and predicate it is customary to employ the terms in an extensional sense only, since such a restriction serves the purpose of syllogistic reasoning and conversion.