nity is the measure of a permanent being; while time is a measure
of movement. Supposing, however, that the aforesaid difference be
considered on the part of the things measured, and not as regards the
measures, then there is some reason for it, inasmuch as that alone is
measured by time which has beginning and end in time. Hence, if the
movement of the heavens lasted always, time would not be of its
measure as regards the whole of its duration, since the infinite is
not measurable; but it would be the measure of that part of its
revolution which has beginning and end in time.

Another reason for the same can be taken from these measures in
themselves, if we consider the end and the beginning as
potentialities; because, granted also that time always goes on, yet it
is possible to note in time both the beginning and the end, by
considering its parts: thus we speak of the beginning and the end of a
day or of a year; which cannot be applied to eternity. Still these
differences follow upon the essential and primary differences, that
eternity is simultaneously whole, but that time is not so.

Reply Obj. 1: Such a reason would be a valid one if time and eternity
were the same kind of measure; but this is seen not to be the case
when we consider those things of which the respective measures are
time and eternity.

Reply Obj. 2: The "now" of time is the same as regards its subject in
the whole course of time, but it differs in aspect; for inasmuch as
time corresponds to movement, its "now" corresponds to what is
movable; and the thing movable has the same one subject in all time,
but differs in aspect a being here and there; and such alteration is
movement. Likewise the flow of the "now" as alternating in aspect is
time. But eternity remains the same according to both subject and
aspect; and hence eternity is not the same as the "now" of time.

Reply Obj. 3: As eternity is the proper measure of permanent being,
so time is the proper measure of movement; and hence, according as
any being recedes from permanence of being, and is subject to change,
it recedes from eternity, and is subject to time. Therefore the being
of things corruptible, because it is changeable, is not measured by
eternity, but by time; for time measures not only things actually
changed, but also things changeable; hence it not only measures
movement but it also measures repose, which belongs to whatever is
naturally movable, but is not actually in motion.