The Complete Aristotle (eng.)

independently and by itself what, together with a
number of other things, he ought to have shown. Again, a man begs
the question if he begs his conclusion piecemeal: supposing e.g.
that he had to show that medicine is a science of what leads to
health and to disease, and were to claim first the one, then the
other; or, fifthly, if he were to beg the one or the other of a
pair of statements that necessarily involve one other; e.g. if he
had to show that the diagonal is incommensurable with the side, and
were to beg that the side is incommensurable with the diagonal.
    The ways in which people assume contraries are equal in number
to those in which they beg their original question. For it would
happen, firstly, if any one were to beg an opposite affirmation and
negation; secondly, if he were to beg the contrary terms of an
antithesis, e.g. that the same thing is good and evil; thirdly,
suppose any one were to claim something universally and then
proceed to beg its contradictory in some particular case, e.g. if
having secured that the knowledge of contraries is one, he were to
claim that the knowledge of what makes for health or for disease is
different; or, fourthly, suppose him, after postulating the latter
view, to try to secure universally the contradictory statement.
Again, fifthly, suppose a man begs the contrary of the conclusion
which necessarily comes about through the premisses laid down; and
this would happen suppose, even without begging the opposites in so
many words, he were to beg two premisses such that this
contradictory statement that is opposite to the first conclusion
will follow from them. The securing of contraries differs from
begging the original question in this way: in the latter case the
mistake lies in regard to the conclusion; for it is by a glance at
the conclusion that we tell that the original question has been
begged: whereas contrary views lie in the premisses, viz. in a
certain relation which they bear to one another.
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14
    The best way to secure training and practice in arguments of
this kind is in the first place to get into the habit of converting
the arguments. For in this way we shall be better equipped for
dealing with the proposition stated, and after a few attempts we
shall know several arguments by heart. For by ‘conversion’ of an
argument is meant the taking the reverse of the conclusion together
with the remaining propositions asked and so demolishing one of
those that were conceded: for it follows necessarily that if the
conclusion be untrue, some one of the premisses is demolished,
seeing that, given all the premisses, the conclusion was bound to
follow. Always, in dealing with any proposition, be on the look-out
for a line of argument both pro and con: and on discovering it at
once set about looking for the solution of it: for in this way you
will soon find that you have trained yourself at the same time in
both asking questions and answering them. If we cannot find any one
else to argue with, we should argue with ourselves. Select,
moreover, arguments relating to the same thesis and range them side
by side: for this produces a plentiful supply of arguments for
carrying a point by sheer force, and in refutation also it is of
great service, whenever one is well stocked with arguments pro and
con: for then you find yourself on your guard against contrary
statements to the one you wish to secure. Moreover, as contributing
to knowledge and to philosophic wisdom the power of discerning and
holding in one view the results of either of two hypotheses is no
mean instrument; for it then only remains to make a right choice of
one of them. For a task of this kind a certain natural ability is
required: in fact real natural ability just is the power right to
choose the true and shun the false. Men of natural ability can do
this; for by a right liking or disliking for whatever is proposed
to them they rightly select what is best.
    It is best to know by heart arguments upon those questions which
are of most frequent occurrence, and particularly in regard to
those propositions which are ultimate: for in discussing these
answerers frequently give up in despair. Moreover, get a good stock
of definitions: and have those of familiar and primary ideas at
your fingers’ ends: for it is through these that reasonings are
effected. You should try, moreover, to master the heads under which
other arguments mostly tend to fall. For just as in