The Complete Aristotle (eng.)

ambiguous terms, for there is a
difference between what is prior and better known in the order of
being and what is prior and better known to man. I mean that
objects nearer to sense are prior and better known to man; objects
without qualification prior and better known are those further from
sense. Now the most universal causes are furthest from sense and
particular causes are nearest to sense, and they are thus exactly
opposed to one another. In saying that the premisses of
demonstrated knowledge must be primary, I mean that they must be
the ‘appropriate’ basic truths, for I identify primary premiss and
basic truth. A ‘basic truth’ in a demonstration is an immediate
proposition. An immediate proposition is one which has no other
proposition prior to it. A proposition is either part of an
enunciation, i.e. it predicates a single attribute of a single
subject. If a proposition is dialectical, it assumes either part
indifferently; if it is demonstrative, it lays down one part to the
definite exclusion of the other because that part is true. The term
‘enunciation’ denotes either part of a contradiction indifferently.
A contradiction is an opposition which of its own nature excludes a
middle. The part of a contradiction which conjoins a predicate with
a subject is an affirmation; the part disjoining them is a
negation. I call an immediate basic truth of syllogism a ‘thesis’
when, though it is not susceptible of proof by the teacher, yet
ignorance of it does not constitute a total bar to progress on the
part of the pupil: one which the pupil must know if he is to learn
anything whatever is an axiom. I call it an axiom because there are
such truths and we give them the name of axioms par excellence. If
a thesis assumes one part or the other of an enunciation, i.e.
asserts either the existence or the non-existence of a subject, it
is a hypothesis; if it does not so assert, it is a definition.
Definition is a ‘thesis’ or a ‘laying something down’, since the
arithmetician lays it down that to be a unit is to be
quantitatively indivisible; but it is not a hypothesis, for to
define what a unit is is not the same as to affirm its
existence.
    Now since the required ground of our knowledge-i.e. of our
conviction-of a fact is the possession of such a syllogism as we
call demonstration, and the ground of the syllogism is the facts
constituting its premisses, we must not only know the primary
premisses-some if not all of them-beforehand, but know them better
than the conclusion: for the cause of an attribute’s inherence in a
subject always itself inheres in the subject more firmly than that
attribute; e.g. the cause of our loving anything is dearer to us
than the object of our love. So since the primary premisses are the
cause of our knowledge-i.e. of our conviction-it follows that we
know them better-that is, are more convinced of them-than their
consequences, precisely because of our knowledge of the latter is
the effect of our knowledge of the premisses. Now a man cannot
believe in anything more than in the things he knows, unless he has
either actual knowledge of it or something better than actual
knowledge. But we are faced with this paradox if a student whose
belief rests on demonstration has not prior knowledge; a man must
believe in some, if not in all, of the basic truths more than in
the conclusion. Moreover, if a man sets out to acquire the
scientific knowledge that comes through demonstration, he must not
only have a better knowledge of the basic truths and a firmer
conviction of them than of the connexion which is being
demonstrated: more than this, nothing must be more certain or
better known to him than these basic truths in their character as
contradicting the fundamental premisses which lead to the opposed
and erroneous conclusion. For indeed the conviction of pure science
must be unshakable.
3
    Some hold that, owing to the necessity of knowing the primary
premisses, there is no scientific knowledge. Others think there is,
but that all truths are demonstrable. Neither doctrine is either
true or a necessary deduction from the premisses. The first school,
assuming that there is no way of knowing other than by
demonstration, maintain that an infinite regress is involved, on
the ground that if behind the prior stands no primary, we could not
know the posterior through the prior (wherein they are right, for
one cannot traverse an infinite series): if on the other hand-they
say-the series