A fortiori is a Latin phrase meaning 'with more reason'. In logic this term is used to refer to a form of argumentation by which one derives a consequence from one thing in view of the conclusion drawn from another, for which there was less reason. For example, if the one who steals is sentenced, a fortiori the one who kills will be sentenced.
It is the degree immediately above the a pari, where the conclusion is formulated with equal force in the terms compared and differs only materially to necessary; the a fortiori ilation merely indicates an excess of reason, of force, and a necessary one expresses the intrinsic binding of the conclusion and the premise, which is for that which the necessary cause is to its effect or the principle to the derivative of it. Like these other forms, it is based on the well-known axioms of the dictum omni schools, dictum of singulis sub eis contentis (quoad extensionem); dictum de nullo, dictum de singulis (quoad extensionem et comprehesionem), which applies with the indicated excess of reason. Specific bibliography
wiki