We must either state the case thus, or else premise that the
conclusion of demonstration is necessary and that a demonstrated
conclusion cannot be other than it is, and then infer that the
conclusion must be developed from necessary premisses. For though
you may reason from true premisses without demonstrating, yet if
your premisses are necessary you will assuredly demonstrate-in such
necessity you have at once a distinctive character of demonstration.
That demonstration proceeds from necessary premisses is also indicated
by the fact that the objection we raise against a professed
demonstration is that a premiss of it is not a necessary truth-whether
we think it altogether devoid of necessity, or at any rate so far as
our opponent's previous argument goes. This shows how naive it is to
suppose one's basic truths rightly chosen if one starts with a
proposition which is (1) popularly accepted and (2) true, such as
the sophists' assumption that to know is the same as to possess
knowledge. For (1) popular acceptance or rejection is no criterion
of a basic truth, which can only be the primary law of the genus
constituting the subject matter of the demonstration; and (2) not
all truth is 'appropriate'.
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