Supposition (Logic)

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The word supposition (Lat. suppositio ) originally meant substitution, and commonly indicates an assumption, hypothesis, or theory. In logic, the notion of substitution is retained in the first meaning of supposition, which is the same as that of signification, that is, "the name stands for the thingnomen supponit pro re." As Aristotle observes, "it is impossible in a discussion to bring in the actual things discussed; we use their names as signs instead of them" (Soph. elen. 165a 5). St. Thomas Aquinas points out that what the name stands for is called the substance of the name, namely, that which underlies the name (In 3 sent. 6.1.3 ).

Supposition and the Proposition. Thus initially understood, supposition takes on a further meaning when one considers the name as part of a proposition. Although the name is made to stand for what is named, whether within or apart from a proposition, yet when it is a part of a proposition and the proposition is to be true, the substance of the name will not be indifferent to the time expressed by the verb. For example, to say that "Caesar is" in the sense of "exists" would be false, since Caesar no longer exists. In other words, there is a discrepancy between the substance of the name "Caesar," which no longer exists but in memory, and the tense of the verb "to be," which here stands as both copula and predicate. On the other hand, to say that "Caesar is praiseworthy" is true, for what is predicated here is not "existence" but "praiseworthy." We thus arrive at the second logical meaning of supposition: the verification of a name in a proposition in accordance with the requirements of the verb copula.

The verb as mere copula must signify with present time. For example, "Caesar was," logically analyzed, implies that it is true (at the present time) that he was. In other words, if a proposition was true in the past (there was a time when it was true to say: "Caesar is"), but now no longer is true, it must nonetheless now be true that it was true. All propositions, whether about the present, the past, or the future, are formed in the present.

The logic of supposition stretches back to ancient Greece. Thus as Aristotle points out, "Homer is something, say, a poet. Is it therefore true to say also that Homer is, or not? The 'is' here is predicated accidentally of Homer, for the 'is' is predicated of him with regard to the fact that he is a poet, not in itself." (Interp. 21a 2528.) In the Middle Ages, the doctrine of supposition was developed extensively. At first, it was discussed in terms of a parallel with grammatical structure (namely, imposition), for example by abelard and john of salisbury. Later it was treated more formally as a distinctive logical doctrine, in extensive detail, by such medieval authors as Peter of Spain (Pope john xxi), william of ockham and john buridan; still later by St. vincent ferrer and john of st. thomas.

Kinds of Supposition. Only some principal kinds of supposition are here mentioned, considering first supposition in a proposition as determined by the way in which the predicate is attributed, and then supposition on the part of the predicate itself.

Material Supposition. This is the use of a word to stand for itself with respect to its oral or written aspect; thus, "Man is a name"; "Man is of one syllable."

Personal Supposition. This is the normal use of names as they stand for subjects of propositions. Thus in the proposition "Man is an animal," "man" is used so as to stand both for what can be said of any individual of the nature signified by the word, and for this nature taken universally. The designation "personal" derives from the more known instance of a name's standing for individual persons as well as for the nature; this meaning is extended to the individuals of any nature and not just to individual human beings, who are persons. Personal supposition is further divided into universal (Every man is wise), particular (Some man is wise), indefinite (Man is wise) and singular (Peter is wise).

Simple Supposition. This is the use of a name to stand for what it immediately signifies, the nature as known, without including the individuals of that nature. Thus in "Man is a species" (whether a natural or predicable species), "man" stands for the nature as known by the mind, excluding the individuals of that nature, for no individual man is a species.

The Predicate. Supposition on the part of the predicate is taken either universally (distributed) or particularly (undistributed), the latter in the sense of "some." Every negative proposition has the predicate standing universally, for in a negation the predicate is always denied universally of the subject. Every affirmative proposition, on the contrary, always has a predicate standing particularly. Thus, in the proposition "Every man is an animal," the predicate cannot be taken universally, otherwise one would assert that every man is every animal and that each man is every animal.

Other divisions of supposition, as well as the relation of supposition to ampliation, restriction, alienation, diminution and appellation, are discussed in logic textbooks (for example, see bibliography).

Relevance of Supposition. The logical doctrine of supposition is as significant now as it has been in the past. To ignore it leads to weird logical paradoxes. The role of supposition calls attention to the fact that words, being restricted to the limitations of a material medium, cannot adequately or fully convey the expression of thought. Indispensable though sense signs and symbols are to man's thinking and the expression of it, nevertheless his thought cannot be identified with a linguistic system. Supposition manifests the suppleness of the human intellect in dealing with words. It brings out the manner in which the mind, while attending to the meaning a word has, can still use the word to stand for various things in a variety of ways (see semantics).

Moreover, sound reasoning depends upon the correct supposition of terms. A valid syllogism, for example, must retain the same supposition of terms throughout; a shift in supposition renders an argument fallacious. The supposition of terms is particularly relevant for the relations between propositions as expressed in the square of opposition. Attempts to invalidate the rules of truth and falsity for contrariety, subcontrariety and subalternation fail to take into account that the supposition of names in propositions must be applied consistently, particularly with respect to the kind of existence signified. Neglect of supposition in logic invites inconsistency in thinking and an oversimplification of the function of language as expressing thought.

It may also be noted that the logic of supposition is particularly crucial in understanding the theology of the Trinity.

See Also: logic; logic, history of; term (logic); proposition.

Bibliography: j. a. oesterle, Logic: The Art of Defining and Reasoning (2d ed. Englewood Cliffs, N.J. 1963). g. giannini, Enciclopedia filosofica, 4 v. (Venice-Rome 1957) 4:105051.

[j. a. oesterle]