A Fortiori Argument Law and Legal Definition

A fortiori argument means an argument from a strong source. This term is derived from the Latin legal maxim argumentum a fortiori. A fortiori argument is applied by following the logic that a point to be proved is followed from a stronger claim. This principle is applied in situations where:

1. a proposition previously given or proven in the argument contains and implies a variety of "weaker" or less contentful materials ; and

2. a proposition being proven is only one of the propositions contained and implied.

There are mainly two types of a fortiori argument. They are:

1. a maiore ad minus, that means from greater to smaller; and

2. a minore ad maius, that means from smaller to greater.