sequence of their being cannot be reversed. In this sense 'one'
is 'prior' to 'two'. For if 'two' exists, it follows directly
that 'one' must exist, but if 'one' exists, it does not follow
necessarily that 'two' exists: thus the sequence subsisting
cannot be reversed. It is agreed, then, that when the sequence of
two things cannot be reversed, then that one on which the other
depends is called 'prior' to that other.

In the third place, the term 'prior' is used with reference to
any order, as in the case of science and of oratory. For in
sciences which use demonstration there is that which is prior and
that which is posterior in order; in geometry, the elements are
prior to the propositions; in reading and writing, the letters of
the alphabet are prior to the syllables. Similarly, in the case
of speeches, the exordium is prior in order to the narrative.

Besides these senses of the word, there is a fourth. That which
is better and more honourable is said to have a natural priority.
In common parlance men speak of those whom they honour and love
as 'coming first' with them. This sense of the word is perhaps
the most far-fetched.

Such, then, are the different senses in which the term 'prior' is
used.

Yet it would seem that besides those mentioned there is yet
another. For in those things, the being of each of which implies
that of the other, that which is in any way the cause may
reasonably be said to be by nature 'prior' to the effect. It is
plain that there are instances of this. The fact of the being of