In essence, this amounts to comparing a thing with its counterpart and subsuming the particular under the universal. The former is called analogical reasoning, and the latter syllogistic reasoning. The two necessarily entail one another: the feature shared by the individual instances in syllogistic reasoning, which logicians call the “middle term,” is the same shared feature in analogical reasoning that scholars of legal theory call the common factor, the basis of the ruling, the effective cause, the indicator, the prompting factor, the motivating factor, the factor calling for the ruling, the factor necessitating it, the shared attribute, and other such expressions.

As for inferring the basis of a ruling, this is analogy proper: a ruling is explicitly laid down concerning certain cases, and one might suppose that it applies exclusively to them. It is then established by inference that other cases are equivalent to them, either because there is no relevant distinction between them, or because they share the attribute to which, as the evidence establishes, the Lawgiver attached the ruling in the original case. This is the analogy accepted by the great majority of scholars and rejected by those who deny analogy.

Errors frequently arise in this because the common factor to which the Lawgiver attached the ruling is not known. This is the issue addressed by what is called the “demand for proof”: the objector requires the proponent of the argument to establish that the attribute shared by the original case and the new case is the effective cause of the ruling, or an indication of that cause. Most errors made by those who reason by analogy arise from supposing something to be the effective cause in the original case when it is not. Hence their frequent denunciations of those who employ unsound analogy.

But if evidence establishes that the distinction is irrelevant, and that there is no difference between the original case and the new case on account of which the Lawgiver would distinguish between the two cases; or if there is