Discourses - Book I   

of demonstrating himself the several things which he has proposed, and
the power of understanding the demonstrations of others, including
of not being deceived by sophists, as if they were demonstrating.
Therefore there has arisen among us the practice and exercise of
conclusive arguments and figures, and it has been shown to be
necessary.
But in fact in some cases we have properly granted the premisses
or assumptions, and there results from them something; and though it
is not true, yet none the less it does result. What then ought I to
do? Ought I to admit the falsehood? And how is that possible? Well,
should I say that I did not properly grant that which we agreed
upon? "But you are not allowed to do even this." Shall I then say that
the consequence does not arise through what has been conceded? "But
neither is it allowed." What then must be done in this case?
Consider if it is not this: as to have borrowed is not enough to
make a man still a debtor, but to this must be added the fact that
he continues to owe the money and that the debt is not paid, so it
is not enough to compel you to admit the inference that you have
granted the premisses, but you must abide by what you have granted.
Indeed, if the premisses continue to the end such as they were when
they were granted, it is absolutely necessary for us to abide by
what we have granted, and we must accept their consequences: but if
the premisses do not remain such as they were when they were
granted, it is absolutely necessary for us also to withdraw from
what we granted, and from accepting what does not follow from the
words in which our concessions were made. For the inference is now not
our inference, nor does it result with our assent, since we have
withdrawn from the premisses which we granted. We ought then both to
examine such kind of premisses, and such change and variation of them,
by which in the course of questioning or answering, or in making the
syllogistic conclusion, or in any other such way, the premisses
undergo variations, and give occasion to the foolish to be confounded,
if they do not see what conclusions are. For what reason ought we to
examine? In order that we may not in this matter be employed in an
improper manner nor in a confused way.
And the same in hypotheses and hypothetical arguments; for it is
necessary sometimes to demand the granting of some hypothesis as a
kind of passage to the argument which follows. Must we then allow
every hypothesis that is proposed, or not allow every one? And if
not every one, which should we allow? And if a man has allowed an
hypothesis, must he in every case abide by allowing it? or must he
sometimes withdraw from it, but admit the consequences and not admit
contradictions? Yes; but suppose that a man says, "If you admit the
hypothesis of a possibility, I will draw you to an impossibility."
With such a person shall a man of sense refuse to enter into a
contest, and avoid discussion and conversation with him? But what
other man than the man of sense can use argumentation and is
skillful in questioning and answering, and incapable of being
cheated and deceived by false reasoning? And shall he enter into the
contest, and yet not take care whether he shall engage in argument not