Luís Duarte d’Almeida – Arguing A Fortiori

Political Theory Research Group seminar series: 24 Feb 2016

Photo: Thinkstock

Photo: Thinkstock

Lawyers and courts frequently deploy a fortiori arguments, but rarely disclose the inferential steps on which they are made. This has created opaqueness in the law, and made it difficult to parse valid from fallacious cases of a fortiori reasoning. In his paper, d’Almeida attempts to build a general framework against which potential cases of a fortiori argument can be tested. Imagine the argument ‘because X refuses cider, X will certainly refuse whisky’. What kind of claim is this? It could mean: X rather dislikes alcohol; X dislikes the alcoholic level of cider; if X dislikes the alcoholic level of cider, X will more dislike the alcoholic level of whisky. In other words, it makes the assumption that X refused cider because of its property of alcohol, and claims because whisky as compared cider has more of that property, we have even firmer reason to believe X would refuse whisky than they would cider. Put  generally, then, a fortiori reasoning identifies a scalar property, finds that one object does not reach a certain threshold on that scale, and infers that because a second object is placed further down that scale than the first, we have firmer reason to believe that the second than the first object does not satisfy that threshold. This provisional model in hand, d’Almeida tackles the distinction between maiore ad minus (from the greater to the lesser) and a minore ad maius (from the lesser to the greater) a fortiori arguments, asks what it could mean to conclude that X dislikes whisky even more than cider, and reflects upon what this model can illuminate as to the practice of the law.

Discussion centred on whether a fortiori arguments really are a distinctive kinds of inference, and not some variation of modus tollens or modus ponens; on whether maiore ad minus and a minore ad maius types of a fortiori argument could be usefully subsumed under a single model; and on the applications of this kind of inference.

Written by Louis Fletcher


Luís Duarte d’Almeida is a Reader in Jurisprudence at the University of Edinburgh.