It is entirely consistent with the major premise to suppose that there are other invalidating circumstances (other possible antecedents) which will give the same consequent, e.g., undue influence over the testator in drafting his will. Thus the denial of this particular circumstance (the insanity of the testator) does not warrant a denial of the consequent (that the will is invalid).

Ruggero J. Aldisert, Agatha D. Aldisert, Logic for Lawyers- A Guide …, loc. 934