A SIMILI argument

The following three Latin nouns cover the field of analogy: similis – similitudo – analogia (which came to Latin from Greek). Their global meaning covers the field(s) of « analogy, similarity, resemblance, comparison (up to identity), example, resemblance … ».
The corresponding terms, argument a simili – argument ex similitudine – argument per analogiam, may broadly cover cases corresponding to other, more specific, argument schemes.

1. Cicero’s “a similitudine”: “Argument based on resemblance or analogy”

Under this heading, Cicero cites the similarity of a case to be judged with another case in which there is no doubt about the decision to be made (where justice has already decided?): it is therefore a kind of precedent (Top. III, 14 p. 393 Hubbell) :

If a man has received by will the usufruct of a house, and the house has collapsed or is in disrepair, the heir (i.e., the remainderman) is not bound to restore or repair it, any more than he would have been bound to replace a slave of whom the usufruct had been bequeathed, if the slave had died ».

The following case allows for a particularly interesting plurality of analyses, or cross-analyses (« On Similarity, » Top. X 41, Hubbell, p. 413) :

If honesty is required of a guardian, a partner, a bailee, and a trustee, it is required of an agent (p. 413). This form of argument, which achieves the desired proof by citing several parallels, is called induction in Greek [epagoge].

This case can be seen as an induction as well as a categorization and / or an application of the gender clause.

With a simili, a similitudine, we remain in the large family of analogy and comparison.

2. Perelman: a simili

Perelman defines the argument a simili or “by analogy” as follows:

A legal proposition being given, which affirms a legal obligation relative to a subject or a class of subjects, this same obligation exists with regard to any other subject or class of subjects having with the first subject (or class of subjects) sufficient analogy so that the reason which determined the rule with respect to the first subject (or class of subjects) is valid with respect to the second subject (or class of subjects). Thus, the fact of having forbidden a traveler to climb on the steps accompanied by a dog leads us to the rule that it is also necessary to forbid it to a traveler accompanied by an equally inconvenient animal. (1979, p. 56)

Applying the a fortiori rule, travelers may be accompanied by an animal “less inconvenient” than a dog (perhaps a cat?)” but not by an animal more inconvenient (a goat?)
As the extension clause “an equally inconvenient animal” shows, the a simili argument relies on categorization mechanisms. It involves the same kind of reasoning as a pari, and the rule of justice.
The terminology seems somewhat overlapping, see Analogy; Genus.