Metaphysical Thoughts: Part 1, Chapter 4.
Concerning Duration and Time.
Because above, we have divided being into being whose essence
involves existence, and being whose essence involves only a
possible existence, there arises the distinction between
eternity and duration. Concerning eternity we will speak at
length below. Here we would only say that it is an attribute under
which we conceive the infinite existence of God.
Duration is an attribute under which we conceive
the existence of created objects so far as they persevere in
their own actuality.
From which it clearly follows that duration is distinguished from
the whole existence of an object only by the reason.
For, however much of duration you take away from any thing,
so much of its existence do you detract from it. In order to
determine or measure this we compare this with the duration of
those objects which have a fixed
and a certain motion, and this comparison is called time.
Therefore, time is not an affect of things but only a mode of
thought or, as we have said, a being of reason; it is a mode of thought
serving to explain duration. It should be noted under duration,
as it will be of use when below we are discussing eternity,
that it is conceived as greater or less, as it were, composed of
parts and then not only as an attribute of existence
but as the very essence of existence.