Aristotle defined a syllogism as “discourse in which, certain things being stated something other than what is stated follows of necessity from their being so” (from The Complete Works of Aristotle: The Revised Oxford Translation, ed. by Jonathan Barnes, 1984, by permission of Oxford University Press). But in practice he confined the term to arguments containing two premises and a conclusion, each of which is a categorical proposition. The subject and predicate of the conclusion each occur in one of the premises, together with a third term (the middle) that is found in both premises but not in the conclusion. A syllogism thus argues that because α and γ are related in certain ways to β (the middle) in the premises, they are related in a certain way to one another in the conclusion.

The predicate of the conclusion is called the major term, and the premise in which it occurs is called the major premise. The subject of the conclusion is called the minor term and the premise in which it occurs is called the minor premise. This way of describing major and minor terms conforms to Aristotle’s actual practice and was proposed as a definition by the 6th-century Greek commentator John Philoponus. But in one passage Aristotle put it differently: the minor term is said to be “included” in the middle and the middle “included” in the major term. This remark, which appears to have been intended to apply only to the first figure, has caused much confusion among some of Aristotle’s commentators, who interpreted it as applying to all three figures.

Aristotle distinguished three different figures of syllogisms, according to how the middle is related to the other two terms in the premises. In one passage, he says that if one wants to prove α of γ syllogistically, one finds a middle β such that either α is predicated of β and β of γ (first figure), β is predicated of both α and γ (second figure), or else both α and γ are predicated of β (third figure). All syllogisms must fall into one or another of these figures.

But there is plainly a fourth possibility, that β is predicated of α and γ of β. Many later logicians recognized such syllogisms as belonging to a separate, fourth figure. Aristotle explicitly mentioned such syllogisms but did not group them under a separate figure; his failure to do so has prompted much speculation among commentators and historians. Other logicians included these syllogisms under the first figure. The earliest to do this was Theophrastus (see below Theophrastus of Eresus), who reinterpreted the first figure in so doing.

Four figures, each with three propositions in one of four forms (A, E, I, O), yield a total of 256 possible syllogistic patterns. Each pattern is called a mood. Only 24 moods are valid, 6 in each figure. Some valid moods may be derived from others by subalternation; that is, if premises validly yield a conclusion of form A, the same premises will yield the corresponding conclusion of form I. So too with forms E and O. Such derived moods were not discussed by Aristotle; they seem to have been first recognized by Ariston of Alexandria (c. 50 bce). In the Middle Ages they were called “subalternate” moods. Disregarding them, there are 4 valid moods in each of the first two figures, 6 in the third figure, and 5 in the fourth. Aristotle recognized all 19 of them.

Following are the valid moods, including subalternate ones, under their medieval mnemonic names (subalternate moods are marked with an asterisk):

First figure: Barbara, Celarent, Darii, Ferio,

*Barbari, *Celaront.

Second figure: Cesare, Camestres, Festino, Baroco,

*Cesaro, *Camestrop.

Third figure: Darapti, Disamis, Datisi, Felapton,

Bocardo, Ferison.

Fourth figure: Bramantip, Camenes, Dimaris, Fesapo,

Fresison, *Camenop.

The sequence of vowels in each name indicates the sequence of categorical propositions in the mood in the order: major, minor, conclusion. Thus, for example, Celarent is a first-figure syllogism with an E-form major, A-form minor, and E-form conclusion.

If one assumes the nonsubalternate moods of the first figure, then, with two exceptions, all valid moods in the other figures can be proved by “reducing” them to one of those “axiomatic” first-figure moods. This reduction shows that, if the premises of the reducible mood are true, then it follows, by rules of conversion and one of the axiomatic moods, that the conclusion is true. The procedure is encoded in the medieval names:

1. The initial letter is the initial letter of the first-figure mood to which the given mood is reducible. Thus, Felapton is reducible to Ferio.
2. When it is not the final letter, s after a vowel means “Convert the sentence simply,” and p there means “Convert the sentence per accidens.”
3. When s or p is the final letter, the conclusion of the first-figure syllogism to which the mood is reduced must be converted simply or per accidens, respectively.
4. The letter m means “Change the order of the premises.”
5. When it is not the first letter, c means that the syllogism cannot be directly reduced to the first figure but must be proved by reductio ad absurdum. (There are two such moods.)
6. The letters b and d (except as initial letters) and l, n, t, and r serve only to facilitate pronunciation.

Thus, the premises of Felapton (third figure) are “No β is an α” and “Every β is a γ.” Convert the minor premise per accidens to “Some γ is a β,” as instructed by the “p” after the second vowel. This new proposition and the major premise of Felapton form the premises of a syllogism in Ferio (first figure), the conclusion of which is “Some γ is not an α,” which is also the conclusion of Felapton. Hence, given Ferio and the rule of per accidens conversion, the premises of Felapton validly imply its conclusion. In this sense, Felapton has been “reduced” to Ferio.

The two exceptional cases, which must be proved indirectly by reductio ad absurdum, are Baroco and Bocardo. Both are reducible indirectly to Barbara in the first figure as follows: Assume the A-form premise (the major in Baroco, the minor in Bocardo). Assume the contradictory of the conclusion. These yield a syllogism in Barbara, the conclusion of which contradicts the O-form premise of the syllogism to be reduced. Thus, given Barbara as axiomatic, and given the premises of the reducible syllogism, the contradictory of its conclusion is false, so the original conclusion is true.

Reduction and indirect proof together suffice to prove all moods not in the first figure. This fact, which Aristotle himself showed, makes his syllogistic the first deductive system in the history of logic.

Aristotle sometimes used yet another method of showing the validity of a syllogistic mood. Known as ekthesis (sometimes translated as “exposition”), it consists of choosing a particular object to represent a term—e.g., choosing one particular triangle to represent all triangles in geometric reasoning. The method of ekthesis is of great historical interest, in part because it amounts to the use of instantiation rules (rules that allow the introduction of an arbitrary individual having a certain property), which are the mainstay of modern logic. The same method was used under the same name also in Greek mathematics. Although Aristotle seems to have avoided the use of ekthesis as much as possible in his syllogistic theory, he did not manage to eliminate it completely. The likely reason for his aversion is that the method involved considering particulars and not merely general concepts. This was foreign to Aristotle’s way of thinking, according to which particulars can be grasped by sense perception but not by pure thought.

While the medieval names of the moods contain a great deal of information, they provide no way by themselves to determine to which figure a mood belongs and so no way to reconstruct the actual form of the syllogism. Mnemonic verses were developed in the Middle Ages for this purpose.

Categorical propositions in which α is merely said to belong (or not) to some or every β are called assertoric categorical propositions; syllogisms composed solely of such categoricals are called assertoric syllogisms. Aristotle was also interested in categoricals in which α is said to belong (or not) necessarily or possibly to some or every β. Such categoricals are called modal categoricals, and syllogisms in which the component categoricals are modal are called modal syllogisms (they are sometimes called “mixed” if only one of the premises is modal).

Aristotle discussed two notions of the “possible”: (1) as what is not impossible (i.e., the opposite of which is not necessary) and (2) as what is neither necessary nor impossible (i.e., the contingent). In his modal syllogistic, the term “possible” (or “contingent”) is always used in sense 2 in syllogistic premises, but it is sometimes used in sense 1 in syllogistic conclusions if a conclusion in sense 2 would be incorrect.

Aristotle’s procedure in his modal syllogistic is to survey each valid mood of the assertoric syllogistic and then to test the several modal syllogisms that can be formed from an assertoric mood by changing one or more of its component categoricals into a modal categorical. The interpretation of this part of Aristotle’s logic and the correctness of his arguments have been disputed since antiquity.

Aristotle’s logic presupposes several principles that he did not explicitly formulate about logical relations between any propositions whatever, independent of the propositions’ internal analyses into categorical or any other form. For example, it presupposes that the principle “If p then q; but p; therefore q” (where p and q are replaced by any propositions) is valid. Such patterns of inference belong to what is called the logic of propositions. Aristotle’s logic is, by contrast, a logic of terms in the sense described above. A sustained study of the logic of propositions came only after Aristotle.

Aristotle’s approach to logic differs from the modern one in various ways. Perhaps the most general difference is that Aristotle did not consider verbs for being, such as einai, as ambiguous between the senses of identity (“Coriscus is Socrates”), predication (“Socrates is mortal”), existence (“Socrates is”), and subsumption (“Socrates is a man”), which in modern logic are expressed by means of different symbols or symbol combinations. In the Metaphysics, Aristotle wrote:

One man and a man are the same thing and existent man and a man are the same thing, and the doubling of words in “one man” and “one existent man” does not give any new meaning (it is clear that they are not separated either in coming to be or in ceasing to be); and similarly with “one.”

Aristotle’s refusal to recognize distinct senses of being led him into difficulties. In some cases the trouble lay in the fact that the verbs of different senses behave differently. Thus, whereas being in the sense of identity is always transitive, being in the sense of predication sometimes is not. If A is identical to B and B is identical to C, it follows that A is identical to C. But if Socrates is human and humanity is numerous, it does not follow that Socrates is numerous. In order to cope with these problems, Aristotle was forced to conclude that on different occasions some senses of einai may be absent, depending on the context. In a syllogistic premise, the context includes the two terms occurring in it. Thus, whether “every B is A” has the force “every B is an existent A” (or, “every B is an A and A exists”) depends on what A is and what can be known about it. Thus, existence was not a distinct predicate for Aristotle, though it could be part of the force of the predicate term.

In a chain of syllogisms, existential force, or the presumption of existence, flows “downward” from wider and more general terms to narrower ones. Hence, in any syllogistically organized science, it is necessary to assume the existence of only the widest term (the generic term) by which the field of the science is delineated. For all other terms of the science, existence can be proved syllogistically.

Aristotle’s treatment of existence illustrates the sense in which his logic is a logic of terms. Even existential force is carried not by the quantifiers alone but also, in the context of a syllogistically organized science, by the predicate terms contained in the syllogistic premises.

Another distinctive feature of Aristotle’s way of thinking about logical matters is that for him the typical sentences to which logical rules are supposed to apply are temporally indefinite. A sentence such as “Socrates is sitting,” for example, involves an implicit reference to the moment of utterance (“Socrates is now sitting”), so the same sentence can be both true at one moment and false at another, depending on what Socrates happens to be doing at the time in question. This variability in truth or falsehood is not found in sentences that make explicit reference to an absolute chronology, as does “Socrates is sitting at 12 noon on June 1, 400 bce.”

Aristotle’s conception of logical sentences as temporally indefinite helps explain the intriguing discussion in chapter 9 of De interpretatione concerning whether true statements about the future—e.g., “There will be a sea battle tomorrow”—are necessarily true (because all events in the world are determined by a series of efficient causes). Aristotle’s answer has been interpreted in many ways, but the simplest interpretation is to take him to be saying that, understood as a temporally indefinite statement about the future, “there will be a sea battle tomorrow,” even if true at a certain time of utterance, is not necessary, because at some other time of utterance it might have been false. However, understood as a temporally definite statement—e.g., as equivalent to “there will be a sea battle on June 1, 400 bce”—it is necessarily true if it is true at all, because the battle, like all events in the history of the universe, was causally determined to occur at that particular time. As Aristotle expressed the point, “What is, necessarily is when it is; but that is not to say that what is, necessarily is without qualification [haplos].”