[2]
It has generally been laid down that an argument
to be effective must be based on certainty; for it is
obviously impossible to prove what is doubtful by
what is no less doubtful. Still some things which are
adduced as proof require proof themselves. “You
killed your husband, for you were an adulteress.”1
Adultery must first be proved: once that is certain it
can be used as an argument to prove what is uncertain. “Your javelin was found in the body of the
murdered man.” He denies that it was his. If this
point is to serve as a proof, it must itself be proved.
It is,
1 cp. v. xi. 39.
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