Astronomy too proceeds in the same way.
For indeed every demonstrative science has three elements: (1) that
which it posits, the subject genus whose essential attributes it
examines; (2) the so-called axioms, which are primary premisses of its
demonstration; (3) the attributes, the meaning of which it assumes.
Yet some sciences may very well pass over some of these elements; e.g.
we might not expressly posit the existence of the genus if its
existence were obvious (for instance, the existence of hot and cold is
more evident than that of number); or we might omit to assume
expressly the meaning of the attributes if it were well understood. In
the way the meaning of axioms, such as 'Take equals from equals and
equals remain', is well known and so not expressly assumed.
Nevertheless in the nature of the case the essential elements of
demonstration are three: the subject, the attributes, and the basic
premisses.
That which expresses necessary self-grounded fact, and which we must
necessarily believe, is distinct both from the hypotheses of a science
and from illegitimate postulate-I say 'must believe', because all
syllogism, and therefore a fortiori demonstration, is addressed not to
the spoken word, but to the discourse within the soul, and though we
can always raise objections to the spoken word, to the inward
discourse we cannot always object.
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