 An inference is formally valid in the universal sense if the opposite of its consequent is formally incompatible with its antecedent and a similar form [of argument] holds in all applications (consimilis forma valet in omni materia) (…) An inference is [formally] valid in the restricted sense if the opposite of its consequent is formally incompatible with its antecedent, but not every such application is valid. ([IHT] arg. 11)