Moreover, in establishing
a case, the reasoning brought forward must be universal: for the
definition put forward must be predicated of everything of which the
term is predicated, and must moreover be convertible, if the
definition rendered is to be peculiar to the subject. In
overthrowing a view, on the other hand, there is no longer any
necessity to show one's point universally: for it is enough to show
that the formula is untrue of any one of the things embraced under the
term.
Further, even supposing it should be necessary to overthrow
something by a universal proposition, not even so is there any need to
prove the converse of the proposition in the process of overthrowing
the definition. For merely to show that the definition fails to be
predicated of every one of the things of which the term is predicated,
is enough to overthrow it universally: and there is no need to prove
the converse of this in order to show that the term is predicated of
things of which the expression is not predicated. Moreover, even if it
applies to everything embraced under the term, but not to it alone,
the definition is thereby demolished.
The case stands likewise in regard to the property and genus of a
term also.
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