The Complete Aristotle (eng.)

fully itself when its
correlate is more fully itself’: and ‘there exists a genuine
opinion-in-itself, which will be “opinion” in a more accurate sense
than the particular opinions’: and it has been postulated both that
‘a genuine opinion-in-itself exists’, and that ‘x-in-itself is more
fully x than anything else’: therefore ‘this will be opinion in a
more accurate sense’. Wherein lies the viciousness of the
reasoning? Simply in that it conceals the ground on which the
argument depends.
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    An argument is clear in one, and that the most ordinary, sense,
if it be so brought to a conclusion as to make no further questions
necessary: in another sense, and this is the type most usually
advanced, when the propositions secured are such as compel the
conclusion, and the argument is concluded through premisses that
are themselves conclusions: moreover, it is so also if some step is
omitted that generally is firmly accepted.
    An argument is called fallacious in four senses: (1) when it
appears to be brought to a conclusion, and is not really so-what is
called ‘contentious’ reasoning: (2) when it comes to a conclusion
but not to the conclusion proposed-which happens principally in the
case of reductiones ad impossibile: (3) when it comes to the
proposed conclusion but not according to the mode of inquiry
appropriate to the case, as happens when a non-medical argument is
taken to be a medical one, or one which is not geometrical for a
geometrical argument, or one which is not dialectical for
dialectical, whether the result reached be true or false: (4) if
the conclusion be reached through false premisses: of this type the
conclusion is sometimes false, sometimes true: for while a false
conclusion is always the result of false premisses, a true
conclusion may be drawn even from premisses that are not true, as
was said above as well.
    Fallacy in argument is due to a mistake of the arguer rather
than of the argument: yet it is not always the fault of the arguer
either, but only when he is not aware of it: for we often accept on
its merits in preference to many true ones an argument which
demolishes some true proposition if it does so from premisses as
far as possible generally accepted. For an argument of that kind
does demonstrate other things that are true: for one of the
premisses laid down ought never to be there at all, and this will
then be demonstrated. If, however, a true conclusion were to be
reached through premisses that are false and utterly childish, the
argument is worse than many arguments that lead to a false
conclusion, though an argument which leads to a false conclusion
may also be of this type. Clearly then the first thing to ask in
regard to the argument in itself is, ‘Has it a conclusion?’; the
second, ‘Is the conclusion true or false?’; the third, ‘Of what
kind of premisses does it consist?’: for if the latter, though
false, be generally accepted, the argument is dialectical, whereas
if, though true, they be generally rejected, it is bad: if they be
both false and also entirely contrary to general opinion, clearly
it is bad, either altogether or else in relation to the particular
matter in hand.
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    Of the ways in which a questioner may beg the original question
and also beg contraries the true account has been given in the
Analytics:’ but an account on the level of general opinion must be
given now.
    People appear to beg their original question in five ways: the
first and most obvious being if any one begs the actual point
requiring to be shown: this is easily detected when put in so many
words; but it is more apt to escape detection in the case of
different terms, or a term and an expression, that mean the same
thing. A second way occurs whenever any one begs universally
something which he has to demonstrate in a particular case: suppose
(e.g.) he were trying to prove that the knowledge of contraries is
one and were to claim that the knowledge of opposites in general is
one: for then he is generally thought to be begging, along with a
number of other things, that which he ought to have shown by
itself. A third way is if any one were to beg in particular cases
what he undertakes to show universally: e.g. if he undertook to
show that the knowledge of contraries is always one, and begged it
of certain pairs of contraries: for he also is generally considered
to be begging