begotten of his father; which
implies priority and posteriority of time. Thus it is clear that
equality of nature or of time is absent in every mode whereby one
thing is from another. So if the Son is from the Father, we must say
that He is less than the Father, or later than the Father, or both.

Obj. 2: Further, everything that comes from another has a principle.
But nothing eternal has a principle. Therefore the Son is not
eternal; nor is the Holy Ghost.

Obj. 3: Further, everything which is corrupted ceases to be. Hence
everything generated begins to be; for the end of generation is
existence. But the Son is generated by the Father. Therefore He
begins to exist, and is not co-eternal with the Father.

Obj. 4: Further, if the Son be begotten by the Father, either He is
always being begotten, or there is some moment in which He is
begotten. If He is always being begotten, since, during the process
of generation, a thing must be imperfect, as appears in successive
things, which are always in process of becoming, as time and motion,
it follows that the Son must be always imperfect, which cannot be
admitted. Thus there is a moment to be assigned for the begetting of
the Son, and before that moment the Son did not exist.

_On the contrary,_ Athanasius declares that "all the three persons are
co-eternal with each other."

_I answer that,_ We must say that the Son is co-eternal with the
Father. In proof of which we must consider that for a thing which
proceeds from a principle to be posterior to its principle may be due
to two reasons: one on the part of the agent, and the other on the
part of the action. On the part of the agent this happens differently
as regards free agents and natural agents. In free agents, on account
of the choice of time; for as a free agent can choose the form it
gives to the effect, as stated above (Q. 41, A. 2), so it can choose
the time in which to produce its effect. In natural agents, however,
the same happens from the agent not having its perfection of natural
power from the very first, but obtaining it after a certain time; as,
for instance, a man is not able to generate from the very first.
Considered on the part of action, anything derived from a principle
cannot exist simultaneously with its principle when the action is
successive. So, given that an agent, as soon as it exists, begins to
act thus, the effect would not exist in the same instant, but in the
instant of the action's term