The fundamental expression of argumentative coherence or consistency is non-contradiction.

S. Non-contradiction; Absurd; Ad hominem.

The consistency requirement is of special importance in systems of regulations of human behavior, religion, law, as well as ordinary institutional or familial rulings.

The consistency requirement is expressed a contrario in the refutation strategy mentioned in Aristotle’s Rhetoric, topic n° 22:

Another line of argument is to refute your opponent’s case by noting any contrast or contradiction of dates, acts or words that it anywhere displays. (1400a15; RR p. 373).

1. After the event as before

The topos ≠5, “On consideration of time” appeals to consistency. This topic is not explicitly stated, but presented through two examples:

If before doing the deed I had bargained that, if I did it, I should have a statue, you would have given me one. Will you not give me one now that I have done the deed? (Rhet, II, 23, 5; RR, p. 361).

The situation is this:

    1. X (asks nothing and) accomplishes a feat (maybe an impulsive heroic act)
    2. After this, he asks for a reward.
    3. Argument: if he had asked before, they would have agreed on a reward.

The hero considers that all feats must be paid for as such. It is as if the definition of the word feat includes the characteristic “deserves a reward”:

L1:   — If you do, you’ll receive…
L2:   — I have done and done well, so give me…

This topic explains the disappointment of one who reports the found wallet and receives no reward.

2. Human (in)consistency

Consistency may be the rule, but inconsistency is a fact of life. This is what topic n° 18 expresses:

Men do not always make the same choice on a later and on an earlier occasion, but reverse their previous choice. (Rhet, II, 23, 18; RR, p. 371)

This topic materializes in the following enthymeme:

When we were exiles, we fought in order to return; now we have returned, it would be strange to choose exile in order not to have to fight. (ibid.)

The enthymeme seems to assume the following situation. In the past, exiles fought to return home, and they returned; in the current situation, they are suspected of refusing to fight, and preferring exile. They deny the charge by this enthymeme, which is a claim of consistency, as in:

You fought for this position, now you can’t accept being thrown out like that!

This is a kind of positive ad hominem argument; it may presuppose an a fortiori: “We fought to return to our homeland, a fortiori we will fight to not be chased out of it!
Those accusing them reply that “Men do not always make the same choice, etc.”

The opposing party argues from an opposing vision of human nature; the two opinions “men are constant / inconstant”, are equally probable (see ibid I, 2, 14; p. 25). They can thus be the basis for two antagonistic conclusions.

S. Ad hominem; A fortiori.

3. Consistency of the system of laws
and stability of the objects of the law

Lat. arg. a cohærentia, de cohærentia, “formation into a compact whole”.

3.1 Principle of coherence of laws, a cohærentia

This principle requires that, within a legal system, one norm cannot conflict with another; the system does not allow antinomies. An argumentative line can therefore be rejected if it leads to the view that two laws are contradictory; this is a form of argumentation from the absurd.

In practice, this principle excludes the possibility of the same case being settled in two different ways by the courts.

By applying this principle, if two laws contradict one another, they are said to do so only in appearance, and, as a consequence, they must be interpreted so as to eliminate the contradiction. If one of these laws is obscure, it must be clarified by reference to a less doubtful one.

The argument a cohærentia is used to solve conflicts of standards. To prevent this kind of conflict, the legal system provides for adages, which are interpretative meta-principles, such as “the most recent law takes precedence over the oldest”. These adages are interpretative meta-principles, coming from Roman law and sometimes expressed in Latin: “lex posterior derogat legi priori”.

3.2 Principle of stability of the object of the law, in pari materia

Lat. in pari materia: lat. par, “equal, like”; materia, “topic, subject” argument “in a similar case, on the same subject”.

The argument a cohærentia deals with the formal non-contradiction of laws in a legal system. The argument in pari materia, or argument “on the same subject”, expresses a substantial form of consistency. It requires that a law be understood in the context of other laws having the same goal or relating to the same subjects, that is to say the same beings (persons, things, acts) or the same topic.

The given definition of the subject of the law must be stable and consistent. The application of the argumentation a pari presupposes the stability of the legal categories. S. Classification; A pari.
This principle of consistency prompts the legislator to harmonize the system of laws on the same subject. What constitutes the same subject and the set of laws on the same subject might be questioned. Anti-terrorism laws, for example, are a package of different statutory provisions, for which it is necessary to ensure that the definition of “terrorism” remains the same in each of the passages that uses the term. If this is not the case, these laws need to be made consistent, which implies that they themselves must be underpinned by consistent policy.

The two topoi discussed in the two following paragraphs are taken from Aristotle’s Rhetoric. They are based on the two incompatible, but equally recognized substantial topoi, “human conduct is, or should be consistent” and “human conduct is inconsistent”.

4. Argument from narrative inconsistency

As a particular case of ad hominem argumentation, showing inconsistencies in the accusatory narrative can rebut a charge:

S1:    — you are the heir, you benefit from the crime, you killed to inherit!
S2:    — if so, I should have murdered the other legatees too.

The prosecution will have to prove that S2 also intended to murder the other heir, or otherwise find an alternative motive. The defense starts from the hypothesis put forward by the prosecution to show that the actions of the suspect do not fit in the proposed scenario; the accusatory narrative contains flaws or contradictions.


The argument of incoherent accusation exploits a basic principle of practical rationality: the actions of the suspect must be consistent with his or her claimed goal. The accused can refute the accusatory narrative by showing that, according to this narrative, he acted inconsistently:

You say I’m the murderer. But it has been proven that just before the crime, I spent an hour at the cafe in front of the victim’s home, everyone saw me. It is not coherent conduct on the part of a murderer to show himself at the scene of his crime.

Any weakness identified in the prosecution scenario can then be used to clear the defendant.

The principle of consistency of laws and the principle of stability of the subject of the law concern the coherence of the legal system. The argument from the inconsistency of the narrative exploits the resources of narrative rationality: all the narratives offered as excuses, all the narratives mingled with argumentation are vulnerable to this type of refutation.
Conversely, the argument seems plausible and reasonable because the story is so, and because the speaker knows how to tell it.

The strategies described in the topoi n° 22, 25 and 27 and probably 18 (cf. supra) of the Rhetoric are relevant to this discussion (Aristotle, Rhet., II, 23), S. Collections (2).