Syllogisms

a. Introduction

A categorical syllogism is the inference of one categorical proposition, the conclusion, from two others, the premises, each premise having one term in common with the conclusion and one term in common with the other premises -- for example:

Every animal is mortal;
Every man is an animal;
Therefore, every man is mortal.

b. Figures and moods

Syllogisms are divided into four figures, according to the placing of the middle term in the two premises.

In the first figure the middle term is subject in the major premise and predicate in the minor.

In the second figure the middle term is predicate in both.

In the third figure the middle term is subject in both.

In the fourth figure the middle term is predicate in the major premise and subject in the minor.

The following schemata, with P for the major term, S for the minor, and M for the middle, sum up these distinctions:

Figure 1 Figure 2 Figure 3 Figure 4

M - P P - M M - P P - M

S - M S - M M - S M - S

----- ----- ----- -----

S - P S - P S - P S - P

Within each figure, syllogisms are further divided into moods, according to the quantity and quality of the propositions they contain.

Not all of the theoretically possible combinations of propositions related as above constitute VALID syllogisms, sequences in which the third proposition really follows from the other two.

For example:

Every man is an animal

Some horse is an animal

Therefore,

No man is a horse

(mood AIE in figure 2)

The above is completely inconsequent, even though all three propositions happen in this case to be true. During the Middle Ages, those syllogistic moods that are valid acquired certain short names, with the mood indicated by the vowels, and all of them were put together in a piece of mnemonic doggerel, of which one of the later versions is the following:

Barbara, Celarent, Darii, Ferioque prioris;

Cesare, Camestres, Festino, Baroco secundae;

Tertia Darapti, Disamis, Datisi, Felapton,

Bocardo, Ferison habet. Quarta insuper addit

Bramantip, Camenes, Dimaris, Fesapo, Fresison.

So, Bocardo, for example, means the mood OAO in figure 3, of which an illustration would be:

Some patriarch is not mortal;

Every patriarch is a man;

Therefore,

Some man is not mortal.

Without the cute words, the valid moods for each figure are:

Figure 1

AAA, EAE, AII, EIO

Figure 2

EAE, AEE, EIO, AOO

Figure 3

AAI, IAI, AII, EAO, OAO, EIO

Figure 4

AAI, AEE, IAI, EAO, EIO

There is also a group of moods (Barbari and Celaront in Figure 1, Cesaro and Camestrop in Figure 2, and Camenop in Figure 4) in which a merely particular conclusion is drawn although the premises would warrant our going further and making the conclusion universal (the "subaltern" moods). The Ramists added special moods involving singulars (if we write S and N for affirmative and negative singulars, we have ASS and ESN in Figure 1, ANN and ESN in Figure 2 and SSI and NSO in Figure 3).

It may be noted that every syllogism must have at least one universal premise, except for SSI and NSO in Figure 3 -- the so-called "expository syllogisms". Example:

Enoch is not mortal

Enoch is a patriarch,

Therefore,

Not every patriarch is mortal.

Moreover, every syllogism must have at least one affirmative premise, and if either premise is negative or particular, the conclusion must be negative or particular, as the case may be. "The conclusion follows the weaker premise", as Theophratus put it; negatives and particulars are considered weaker than affirmatives and universals.

c. Reduction

The mnemonic verses serve to indicate how the valid moods of the later figures may be "reduced" to those of Figure 1 -- that is, how we may derive their conclusions from their premises without using any syllogistic reasoning of other than the first-figure type. (This amounts, in modern terms, to proving their validity from that of the first-figure moods, taken as axiomatic).

In the second figure mood Cesare, for example, the letter s after the first e indicates that if we simply convert the major premise we will have a pair of premises from which we can deduce the required conclusion in Figure 1, and the initial letter C indicates that the first-figure mood employed will be Celarent. An example of a syllogism in Cesare (EAE in Figure 2) would be:

No horse is a man;

Every psychopath is a man;

Therefore,

No pyschopath is a horse.

This conclusion may eqully be obtained from these premises by proceeding as follows:

No horse is a man s No man is a horse;

Every psychopath is a man Every psychopath is a man;

Therefore,

No psychopath is a horse.

Here the right-hand syllogism, in which the first premise is obtained from the given major by simple conversion and the second is just the given minor unaltered, is in the mood Celarent in the first figure. Festino "reduces" similarly to Ferio and Datisi and Ferison (in the third figure) reduce to Darii and Ferio, though in the third figure cases it is the minor premise that must be simply converted. Darapti and Felapton reduce to Darii and Ferio by conversion of the minor premise, not simply, but per accidens (this is indicated by the s of the other moods being changed to p).

Camestres (Figure 2) and Disamis (Figure 3) are a little more complicated. Here we have not only an s, for the simple conversion of a premise, but also an m, indicated that the premises must be transposed, and a further s at the end because the transposed premises yield, in Figure 1, not the required condlusion but rather its converse, from which the required conclusion must be obtained by a further conversion at the end of the process. An example of Disamis would be the following:

Some men are liars;

All men are automata;

Therefore,

Some automata are liars.

If we convert the major premise and transpose the two, we obtain the new pair:

All men are automata;

Some liars are men.

From these we may obtain in the first-figure mood Darii not immediately the conclusion "Some automata are liars" but rather, "Some liars are automata," from which, however, "some automata are liars" does follow by simple conversion.

Baroco and Bocardo are different again. In both of them neither premise is capable of simple conversion, and if we convert the A premises per accidens we obtain pairs IO and OI, and there are no valid first-figure moods with such premises -- in fact, no valid moods at all with two particular premises. We therefore show that the conclusion follows from the premises by the device called reductio ad absurdum. That is, we assume for the sake of argument that the conclusion does NOT follow from the premises -- i.e., that the premises can be true and the conclusion false -- and from this assumption, using first figure reasoing alone, we deduce impossible consequences. The assumption, therefore, cannot stand, so the conclusion does after all follow from its premises.

Take, for example, the following syllogism in Baroco (AOO in Figure 2):

Every man is mortal;

Some patriarch (viz., Enoch) is not mortal;

Therefore,

Some patriarch is not a man.

Suppose the premises are true and the conclusion is not true. Then we have:

1) Every man is mortal;

2) Some patriarch is not mortal;

3) Every patriarch is a man.

This is contradictory of the conclusion. But from 1) and 3), in the first-figure mood Barbara, we may infer

4) Every patriarch is mortal.

However, the combination of 2) and 4) is impossible. Hence, we can have both 1) and 2) only if we drop 3) -- that is, if we accept the conclusion of the given second-figure syllogism.

It is possible to "reduce" all the second figure and third-figure moods to Figure 1 by this last method, and although this procedure is a little complicated, it brings out better than the other reductions the essential character of second-figure and third-figure reasoning. Figure 1 is governed by what is called the dictum de omni et nullo, the principle that what applies to all or none of the objects in a given class will apply or not apply (as the case may be) to any member or subclass of this class. As Kant preferred to put it, first-figure reasoning expresses the subsumption of cases under a rule -- the major premise states some affirmative or negative rule ("Every man is mortal", "No man will live forever"), the minor asserts that something is a case or some things are cases, to which this rule applies ("Enoch and Elijah are men"), and the conclusion states the result of applying the rule to the given case or cases ("Enoch and Elijah are mortal," "Enoch and Elijah will not live forever"). Hence, in Figure 1 the mjor premise is always universal (that being how rules are expressed) and the minor affirmative ("Something IS a case").

Second-figure reasoning also begins with the statement of a rule ("Every man is mortal") but in the minor premise DENIES that we have with a given example the result which the rule prescribes ("Enoch and Elijah are NOT mortal," "Enoch and Elijah WILL live forever") and concludes that we do NOT have a case to which the rule applies ("Enoch and Elijah cannot be men"). It combines, in effect, the first-figure major with the contradictory of the first figure conclusion to obtain the contradictory of the first-figure minor (compare the "reduction" of Baroco). A second-figure syullogism, in consequence, must have a universal major, premises opposed in quality, and a negative conclusion. Its practical uses are in refuting hypotheses, as in medicine or detection ("Whowever has measles has spots, and thich child has no spots, so he does not have measles"; "Whoever killed X was a person of great strength and Y is not such a person, so Y did not kill X").

In the third figure we begin by asserting that something or other does not exhibit the result which a proposed rule would give ("Enoch and Elijah are NOT mortal," "Enoch and Elijah WILL live forever"), go on to say that we nevertheless DO have here a case or cases to which the rule would apply if true ("Enoch and Elijah ARE mn"), and conlcude that the rule is not true ("Not all men are mortal," "Some men do live forever"). A third-figure syllogism, consequently, has an affirmative minor (the thing IS a case) and a particular conclusion (the contradictory of a universal being a particular); its use is to confute rashly assumed rules, such as proposed scientific laws.

This rather neat system of interrelations (first clearly brought out by C.S. Peirce) concerns ONLY the first three figures; it was not until the later Middle Ages, in fact, that a distict fourth figure was recognized. The common division of figures assumes that we are considering completed syllogisms, with the conclusion (and its subject and predicate) already before us; however, the question Aristotle originally put to himself was not "Which completed syllogisms are valid?" but "Which pairs of premises will yield a syllogistic conclusion?"

Starting at this end, we cannot distinguish major and minor premises as those containing, respectively, the predicate and subject of the conclusion. Artistotle distinguished them, in the first figure, by their comparative comprehensiveness and mentioned what we now call the fourth-figure moods as ood cases in which first-figure premises will yield a conclusion wherein the "minor" term is predicated of the "major". Earlier versions of the mnemonic lines accordingly list the fourth-figure moods with the first-figure ones and (since the premises are thought of as being in the first figure order) give them slightly different names (Baralipton, Celantes, Dabitis, Fapesmo, Frisesomorum).

d. Distribution of Terms

Terms may occur in A-, E- , I- , and O-propostions as distributed or as undistributed. The rule is that universals distribute their subjects and particulars distribute their predicates, but what this means is seldom very satisfactorily explained.

It is often said, for example, that a distributed term refers to all, and an undistributed term to only a part, of its extension. BGut in what way does "Some men are mortal," for example, refer to only a part of the class of men? Any man whatever will do to verify it; if any man whatever turns out to be mortal, "Some men are mortal" is true. What the traditional writers were trying to express seems to be something of the following sort: a term t is distributed in a proposition f(t) if and only if it is replaceable in f(t), without loss of truth, by any term "falling under it" in the way that a species falls under a genus. Thus, "man" is distributed in:

Every man is an animal;

No man is a horse;

No horse is a man;

Some animal is not a man.

Why? Because these respectively imply, say,

Every blind man is an animal;

No blind man is a horse;

No horse is a blind man;

Some animal is not a blind man.

On the other hand, it is undistributed in:

Some man is keen-sighted;

Some man is not disabled;

Every Frenchman is a man;

Some keen-sighted animal is a man,

since these do NOT respectively imply

Some blind man is keen-sighted;

Some blind man is not disabled;

Every Frenchman is a blind man;

Some keen-sighted animal is a blind man.

In this sense A- and E-propositions do distribute their subjects and E- and O-propositions their predicates. John Anderson pointed out that the four positive results above may be established syllogistically, given that all the members of a speices (using the term widely) are members of its genus -- in the given case, that all blind men are men. From "Every man is an animal" and "Every blind man is a man", Every blind man is an animal" follows in Barbara; with the second example the syllogism is in Celarent, with the third in Camestres, with the fourth in Baroco. Note, however, that the mere prefixing of "every" to a term is not in itself sufficient to secure its "distribution" in the above sense; for example "man" is not distributed in "Not every man is disabled," since this does not imply "Not every blind man is disabled."

For a syllogism to be valid the middle term must be distributed at least once, and any term distributed in the conclusion must be distributed in its premise (although there is no harm in a term's being distributed in its premise but not in the conclusion). Many syllogisms can quickly be shown to be fallacious by the applications of these rules.

Every man is an animal;

Every horse is an animal;

Therefore,

Every horse is a man.

The above syllogism fails to distribute the middle term "animal," and it is clear that any second figure syllogism with two affirmative premises would have the same fault (since in the second figure the middle term is predicate twiche, and affirmatives do not distribute their predicates).

Oteher special rules for the different figures, such as that in Figures 1 and 3, the minor premise must be affirmative, can be similarly proved from the rules of distribution together with the rules of quality (that a valid syllogism does not have two negative premises and that a conclusion is negative if and only if one premise is). Logicians have endeavored to prove some of these rules from others and to reduce the number of unproved rules to a minimum.

e. Euler's diagrams

One device for checking the validity of syllogistic inferences is the use of certain diagrams attributed to the seventeenth-centruy mathematician Leonhard Euler, although their accurate employment seems to date rather from J.D. Gergonne, in the early nineteenth century.

From the traditional laws of opposition and conversion it can be shown that the extensions of any pair of terms X, Y will be related in one or another of five ways:

Alpha: every X is a Y and every Y is an X. That is, their extensions coincide

Beta: every X is a Y, but not every Y is an X. That is, the X's form a proper part of the Y's.

Gamma: every Y is an X, but not every X is a Y. That is, the Y's form a proper part of the X's.

Delta: Some, but not all, X's are Y's and some, but not all Y's are X's. That is, the X's and Y's overlap.

Epsilon: No X's are Y's and so no Y's are X's. That is, the Sy's and Y's are mutually exclusive.

X, Y X Y Y X

X Y X Y

"Every X is a Y" (A) is true if and only if we have either Alpha or Beta.

"Some X is not a Y" (O) if and only if we have either Gamma or Delta or Epsilon.

"No X is a Y" (E) if and only if we have Epsilon.

"Some X is a Y" (I) if and only if we have either Alpha or Beta or Gamma or Delta.

From these facts it follows that A andO are in no case true together and in no case false together, and similarly for E and I; that I si true in every case in which A is and also in two cases in which A is not, and similarly for O and E; that A and E are in no case true togethert but in two cases are both false; and that O and I are in no case both fasle but in two cases are both true. After working out analogous truth conditions for the forms with reversed terms, we will see that they are the same for the two I's and the two E's (showing that these are simply convertible) but not for th two A's and the two O's (showing that these are not).

Given which of the five relations holds between X and Y and which between Y and Z, we can work out by compounding diagrams what will be the possible relations between X and Z. For example, if we know that every X is a Y and every Y a Z, then we must have either (Alpha)XY and (Alpha)YZ or (Alpha)XY and (Beta)YZ or (Beta)XY and (Alpha)YZ or (Beta)XY and (Beta)Yz; that is we must have:

X,Y,Z Z

or X,Y

(i) (ii)

Y,Z Z

or X or X Y

(iii) (iv)

Inspection will show that for X and Z we have in every case either

X,Z Z

or X

so in every case every X is Z. Hence, Barbara is valid.

When employing this procedure it is essential to consider all the possible cases involved. Barbara is not validated, for example, by donsidering case (iv) alone, as popular expositions of this method sometimes suggest.

f. Polysyllogisms, enthymemes, and induction

1) Polysyllogisms

In an extended argument the conclusion of one inference may be used as a premise of another, and the conclusion of that as premise of a third, and so on. In presenting such an argument we may simply omit the intermediate steps and list all the premises together.

For example, the sequence of categorical syllogisms:

Every X is a Y

Every Y is a Z

Therefore

Every X is a Z

Every Z is a T

Therefore

Every X is a T

may be condensed to

Every X is a Y

Every Y is a Z

Every Z is a T

Therefore

Every X is a T

Such a condensed chain of syllogisms is called a polysyllogism or sorites.

The theory of chains of two syllogisms was thoroughly studied by Galen, as reported in an ancient passage recently unearthed by Jan Lukasiewicz. Galen showed that the only combinations of the Aristotelian three figrues that could be thus used were 1 and 1, 1 and 2, and 1 and 3, and 2 and 3. His discovery of these four types of compound syllogism was misunderstood by later writers as an anticipation of the view that SINGLE syllogisms may be of four figures.

2) Enthymemes

Even when it is not a conclusion from other premises already stated, one of the premises of an inference may often be informally omitted (for example:

Enoch and Elijah are men,

Therefore

Enoch and Elijah are mortals.

Such a truncated inference is often called an enthymeme. This is not Aristotle's own use of the term, though he did mention that a premise is often omitted in the statement of an enthymeme in his sense. An Aristotelian enthymeme is a merely probable argument -- this is, one in which the conclusion does not strictly follow from the premises but is merely made more likely by them. When the claim made for an argument is thus reduced, the normal rules may be relaxed in certain directions; in particular, the second and third figures may be used to yield more than merely negative results. Thus Figure 2 may be used not only to prove that something is not a case falling under a given rule but also to suggest that it is one --to use a modern example:

Any collection of particles whose movement is accelerated will occupy more space that it did;

A heated gase will occupy more space than it did;

Therefore:

A heated gas may be a collection of particles whose movement is accelerated.

Figure 3 may be similarly used not only to prove that some rule does not hold universally but also to suggest that it does hold universally -- for instance:

X,Y,Z are all of them white;

X,Y,Z are all of them snowflakes;

Therefore, perhaps all snowflakes are white.

If the second premise here is strengthened to "X,Y,Z are all the snowflakes there are", the conclusion will follow without any "perhaps" (of course, the new premise in this case is a false one, and the conclusion is also false). The form of inference:

X,Y,Z, etc., are all of the P's;

X,Y,Z, etc., are all the S's there are;

Therefore all S's are P's

was called by Aristotle "induction"; more accurately, he used this term for a similar passage from all the subspecies to their genus ("The X's the Y's and the Z's are all of the P's and are all the S's; therefore..."). He observed that the "conversion" of the second premise to "All the S's are the X's, the Y's, and the Z's" will turn such an induction into a syllogism in Barbara.

The term "induction" being extended in the more recent tradition to cover the merely probable inference given just previously, we distinguish Aristotelian induction by calling it "formal" or "perfect" induction or (as W.E. Johnson called it) "summary" induction. The Figure 2 type of merely probable inference is one of the things meant by the term "argument from" -- or "by" -- "analogy" (or just "analogy"). C.S. Peirce called it "hypothesis".

Summary

The nineteen fundamental types of logical arguments have been given names to make them easier to remember. As a further memory aid, they have been incorporated into a Latin verse:

Barbara, Celarent, Darii, Ferioque, prioris;

Cesare, Camestres, Festino, Baroco, secundae;

Tertia, Darapti, Disamis, Datisi, Felapton, Bocardo, Ferison, habet;

Quaera insuper addit Bramantip, Camenes, Dimaris, Fesapo, Fresison.

It should be noted that when dealing with Darapti, Felapton, Bramantip and Fesapo, we must examine the question of existential import before deciding the validity or invalidity of the syllogism in question.

Rule One: A valid standard-form categorical syllogism must contain exactly three terms, each of which is used in the same sense throughout the argument.

Rule Two: In a valid standard-form categorical syllogism, the middle term must be distributed in at least one premise.

Rule Three: In a valid standard-form categorical syllogism, if either term is distributed in the conclusion, then it must be distributed in the premises.

Rule Four: No standard-form categorical syllogism is valid which has two negative premises.

Rule Five: If either premise of a valid standard-form categorical syllogism is negative, the conclusion must be negative.

Rule Six: No valid standard-form categorical syllogism with a particular conclusion can have two universal premises.

Rule Seven: From two premises beginning with "some", no conclusion can be drawn.

g. Skeptical criticism of syllogistic reasoning

In the latter part of the nineteenth century, under the influence of J.S. Mill, textbooks of the traditional type came to have two main divisions, "formal" or "deductive" logic (dealt with more or less as above) and "inductive" logic or "scientific method". With the details of inductive logic we are not concerned here, but we may glance at the view of some writers that merely probable induction and analogy are the only genuine types of reasoning, "formal" or syllogistic reasoning being useless or spurious because it is inevitably circular, assuming in the premises what it sets out to prove as the conclusion.

The second-century skeptic Sextus Empiricus suggested that, in the syllogism

Every man is an animal

Socrates is a man

therefore

Socrates is an animal

the only way to establish the major premise is by induction. However, if the induction is incomplete the examination of a new instance -- e.g. of Socrates -- might prove it false, and if it is complete the conclusion ("Socrates is an animal") must already have been used in establishing it. This argument was repeated by such writers as George Campbell, in the eighteenth century, who supplemented it with another, to cover the case in which the major is established not by induction but simply by definition or linguistic convention:

"Of course every man is an animal, for bein an animal is part of what we mean by being a man." In this case it is the minor premise, "Socrates is a man," that cannot be established without first establishing the conclusion (that he is an animal). The same point was urged by another Scottish philosopher, Thomas Brown. It is allied to an argument used by Sextus to show not that syllogism is circular but that the major premise is superfluous. If, he said, every man is an animal because it FOLLOWS from an object's being a man that it is an animal, then the allegedly enthymematic "Socrates is a man; therefore, Socrates is an animal" must be valid as it stands.

Richard Whately, answering Campbell's arguments in the early nineteenth century, complained that Campbell had confined himself to examples in which the syllogistic argument was indeed superfluous and countered them with some in which it was not -- for example, the case of some laborers, ignorant of the fact that all horned animals are ruminant, digging up a skeleton whicch they, but not a distant naturalist, coulsd see to be horned, the laaborers and the naturalist thus separately providing premises which were both required to obtain the conclusion that the skeleton was of a ruminant animal. Whately admitted that the sense in which we may make a "discovery" by drawing a syllogistic conclusion is different from that in which we make a discovery by observation, but it can be a genuine discovery none the less; there are "logical" as well as "physical" discoveries.

After Whately, J.S. Mill took up the argument, but it is not entirely clear what side he was on. Sometimes he treated a universal major as already asserting, among other things, the conclusion:

Whoever pronounces the words, All men are mortal, has affirmed that Socrates is mortal, though he may never have heard of Socrates; for since Socrates, whether known to be so or not, really is a man, he is included in the words, All men, and in every assertion of which they are the subject (System of Logic, Book II, Ch. 3, p. 8, note)
"Included in the meaning of the words," he must have meant (for it is obvious that neither Socrates the man nor "Socrates", his name, forms any part of the words "All men"), but this contradicts Mill's own insistence that the meaning of general terms like "men" lies wholly in their "connotation" and that "All men are mortal" means that wherever the attributes of humanity are present, mortality is present, too. He rightly chided Brown, who thought that the meaning of "Socrates is mortal" (like that of "Socrates is an animal") is already contained in the minor premise "Socrates is a man," for failing to distinguish the actual connotation of "man" (i.e., the attributes by which its application is determined) from other attributes (such as mortality) which we may empirically discover these to be attended with, but his own view in the passage cited is similarly negligent.
Mill's main point, however, is different and more defensible. When careful and extensive observation warrants the conclusion that, say, all men are mortal, and we then observe that the duke of Wellingtonis a man and conclude that he is therefore mortal, we have in effect an induction followed by a syllogism. Mill pointed out that if this procedure is justified at all, the introduction of the syllogistic major is superfluous. For if the original body of evidence really does warrant the inference that all men are mortal, it is certainly sufficient to warrant the inference that the duke of Wellington is mortal, given that he is a man. In other words, if we really are justified in the move from particular observations to the general proposition, and from there to new particulars, we would be equally justified in moving directly "from particulars to particulars." What the syllogistic major does, Mill argued, is simply to sum up in a single formula the entire class of inferences to new particulars which the evidence warrants. That is, "All men are mortal" means, in effect, that if we ever find anyone to be a man we are justified in inferring, from the observations we have previously amassed, that he is mortal. "The conclulsion is not an inference drawn from the formula" --i.e., from "All men are mortal" thus understood -- "but an inference drawn according to the formula (ibid., p. 4). Mill here anticipated Gilbert Ryle's treatment of "lawlike statements" as "inference licenses" and echoed Sextus' point that it is inconsistent to require that such licenses be added to the premises of the inferences they permit, since what they license is precisely the drawing of the conclusion from those premises.
Mill in fact here shifted the discussion from Sextus' first skeptical "topic" to his second -- from the charge of circularity to the question of what distinguishes a rule of inference from a premise. On this point more was said later in the nineteenth century by C.S. Peirce. Peirce, like Mill, distinguished sharply between the premise or premises from which, and the "leading principle" according to which, a conclusion is drawn. He also noted, as did Mill, that what is traditionally counted as a premise may function in practice as a "leading principle". But it need not, and, indeed what is traditionally counted as a "leading principle" (say the dictum de omni et nullo may sometimes be, conversely, treated in practice as a premise. Certainly, since all men are mortal (leading principle 1), we are justified in inferrint the mortality of Socrates (or the duke of Wellington, or Elijah) from his humainty. But equally, since all members of any class are also members of any class that contains the former as a subclass (leading principle 2), we are justified in inferring the mortality of Socrates from his being a man AND from men's being a subclass of mortals. For the very same reason (that all members of any class are also members of any class that contains the former as a subclass) we are justified in infering the mortality of Socrates from his being a member of a subclass of the class of mortals AND from the membership of any member of a class in all classes of which it is a subclass. In this last example we have one and the same proposition functioning as a premise and as a leading principle in the same inference (not mmerely, like "All men are mortal" in the preceding two examples, as a leading principle in one and a premise in another); to be capable of this, Peirce thought, is the mark of a "logical" leading principle.
It is not certain that Peirce's method of distinguishing "logical" from other sorts of "leading principles" will bear inspection. However, he seems to have established his basic point, that what it would be fatal to require in all cases -- the treatment of a leading principle as a premise -- we may safely permit in some. There may be useful and valid reasoning about subjects of all degrees of abstraction, including logic itself.
h. Hypothetical and disjunctive syllogisms
Traditional textbooks, aside from developing the theory of categorical propositions and syllogisms, have a brief appendix mentioning "hypothetical" (or "conditional") and "disjunctive" propositions and certain "syllogisms" to which they give rise.
"Hypothetical" syllogisms are divided into "pure", in which premises and conclusion are all of the form "If p then q, and if q then r; therefore, if p then r," analogous to Barbara)
And "mixed" in which only one premise is hypothetical and the other premise and the conclusion are categorical. The mixed hypothetical syllogism has two valid "moods":
1. Modus ponendo ponens:
If p then q, and p; therefore, q.
2. Modus ponendo tollens:
If p then q, but not p; therefore, not q.
In both these moods the hypothetical premise is called the major, the categorical the minor. Ponere, in the mood names, means to affirm, tollere to deny. In (1), by afffirming the antecedent of the hypothetical we are led to affirm its consequent; in (2), by denying its consequent we are led to deny its antecedent. The fallacies of "affirming the consequent" and "denying the antecedent" (i.e., of doing these things to start with, in the minor premise) consist in reversing these procedures -- that is, in arguing "If p then q, and q; therefore, p" and "If p then q, but not q; therefore, not p."

"Disjunctive" syllogisms -- i.e., ones involving "Either-or" propositions -- have the following two "mixed" moods.
3. Modus tollendo ponens:
Either p or q, but not p;
therefore, q (or, butn not q; therefore p).
4. Modus ponendo tollens:
Either p or q, and p;
therefore, not q (or, and q; therefore, not p).
Mood (4) is valid only if "Either p or q is interpreted "exclusively" -- i.e., as meaning "Either p or q but not both" -- whereas (3) is valid even if it is interpreted as "Either p or q or both." There is also a modus tollendo ponens with the simple "Not both p and q" as major and the rest as in (4).

i. Dilemmas
Hypothetical and disjunctive premises may combine to yield a categorical conclusion in the dilemma, or "horned" syllogism (syllogismus cornutus, with its two forms:
5. Constructive:
If p then r, and if q then r, but either p or q; therefore, r.
6. Destructive:
If p then q, and if p then r, but either not q or not r; therefore, not p.
These basic forms have a number of variations; for instance, q in (5) may be simply "not p," making the disjunctive premise the logical truism "Either p or not p"; or p may imply r and q imply s, giving as conclusion "Either r or s" rather than the categorical r; or the disjunctive premise may be conditionalized to "If s then either p or q," making the conclusion "If s then r."
A typical dilemma is that put by Protagoras to Euathlus, whom he had trained as a lawyer on the understanding that he would be paid a fee as soon as his pupil won a case. When the pupil simply engaged in no litigation at all, Protagoras sued him for the fee. His argument was "If Euathlus wins this case, he must pay my fee by our agreement, and if he loses it he must pay it by the judge's decision (for that is what losing this case would mean), but he must either win or lose the case; therefore, in either case he must pay."
"Escaping betweenthe horns" of a dilemma is denying the disjunctive premise; for example, Euathlus might have argued that he would neither win nor lose the case if the judge refused to make any decision. "Taking a dilemma by the horns" is admitting the disjunction but denying one of the implications, as Euathlus might have done by arguing that if he won he would still not be bound by the agreement to pay Protagoras, because this was not the sort of case intended in the agreement. "Rebutting" a dilemma is constructing another dilemma drawing upon the same body of facts but leading to an opposite conclusion. This is what Euathlus did, arguing that if he won the case he would be dispensed from paying by the judge's decision, and if he lost it the agreement would dispense him, so either way he was dispensed from paying. Rebuttal, however, is possible only if one of the other moves (though it may not be clear which) is also possible, for a single set of prremises can lead by equally valid arguments to contradictroy conclusions only if they contain some fault in themselves.
Dilemmatic reasoning obtains a categorical conclusion from hypothetical and disjuctive premises; the Port-Royalists pointed out that we may also obtain hypothetical conclusions from categorical premises. For in any categorical syllogism we may pass directly from one of the premises to the conclusion stated not categorically but conditionally on the truth of the other premise; for instance, from "Every man is mortal" we may infer that IF Socrates is a man he is mortal, and from "Socrates is a man" that if every man is mortal Socrates is, and similarly with all other syllogisms. This "rule of conditionalization" is much used in certain modern logical systems.