Nonne "a fortiori, a priori, a posteriori" solecismi sunt?

Question

Are the terms a fortiori, a priori, and a posteriori bad Latin? If so, how and when did they become established?

I understand that the dative case never takes a preposition in Latin—a most welcome clue in a language where the dative and ablative have opposite meanings but often identical form. But fortiori appears to be unambiguously dative, yet introduced by a preposition. And yet the meaning appears to be ablative: "with stronger reason" ("all the more so" more colloquially in English). The stronger reason is the source of the conclusion being justified. And how would you fill in the elided noun: a fortiori rationi or a fortiore ratione? I don't see how the dative could even make sense here. Nothing is being given to the stronger reason and there’s no suggestion that the stronger reason has anything at stake in the matter. Weight is coming from the stronger reason, as suggested by the preposition a (short for ab as in ablative!).

Answer 1

In classical Latin, the ablative of comparatives could end on -i, although -e is probably more common. Here are a few quotations that I think must be conceded to contain ablatives:

Cornelius Nepos, Vitae Ca. 2.2.2:

… ibi cum diutius moraretur, P. Scipio Africanus consul iterum, cuius in priori consulatu quaestor fuerat, uoluit eum de prouincia depellere …

L. Iunius Moderatus Columella, De Re Rustica 2.1.7.6:

… de cuius cultu dicturos nos priori uolumine polliciti iam nunc disseremus.

Titus Livius, Ab Urbe Condita 24.12.8.3:

… et ceteri in suas quisque prouincias profecti, et quibus prorogatum imperium erat easdem quas priori anno regiones obtinuerunt.

Titus Livius, Periochae Librorum A. U. C. 56.15:

… delatus est ultro Scipioni Africano a senatu populoque R. consulatus; quem cum illi capere ob lege, quae vetabat quemquam iterum consulem fieri, non liceret, sicut priori consulatu legibus solutus est.

Publius Ovidius Naso, Metamorphoses 8.443:

Toxea, quid faciat, dubium pariterque volentem
ulcisci fratrem fraternaque fata timentem
haud patitur dubitare diu calidumque priori
caede recalfecit consorti sanguine telum.

"Still warm from its prior carnage".

Answer 2

My impression is that fortiori, priori and posteriori are ablative forms, but they have been declined badly — from the classical point of view.

Making this mistake is quite easy. Both -e and -ī appear as singular ablative endings in the third declensions. For adjectives, -ī is always used in positive (ie. neither comparative nor superlative) and -e always in comparative. Therefore it is no surprise if by (false) analogy the ablative ending -ī makes it to the comparative as well.

As you explain in your question, ablative makes semantically much more sense than dative. And of course it fits better with the preposition.

From a classical point of view the expressions you mention are bad Latin. I suspect that such declensions may have been canonical in some form of medieval Latin, but I am not an expert in such medieval developments. It seems most natural that this misdeclension is a part of a more wide-spread slip from classical standards, not a stand-alone mistake.