enumerating the definite
properties which secure this result, I have enumerated them in my
recently published book[6] to which I have already referred. Furthermore
the passage of nature enables us to know that one direction along the
series corresponds to passage into the future and the other direction
corresponds to retrogression towards the past.

[6] Cf. _Enquiry_.

Such an ordered series of moments is what we mean by time defined as a
series. Each element of the series exhibits an instantaneous state of
nature. Evidently this serial time is the result of an intellectual
process of abstraction. What I have done is to give precise definitions
of the procedure by which the abstraction is effected. This procedure is
merely a particular case of the general method which in my book I name
the 'method of extensive abstraction.' This serial time is evidently not
the very passage of nature itself. It exhibits some of the natural
properties which flow from it. The state of nature 'at a moment' has
evidently lost this ultimate quality of passage. Also the temporal
series of moments only retains it as an extrinsic relation of entities
and not as the outcome of the essential being of the terms of the
series.

Nothing has yet been said as to the measurement of time. Such
measurement does not follow from the mere serial property of time; it
requires a theory of congruence which will be considered in a later
lecture.

In estimating the adequacy of this definition of the temporal series as
a formulation of experience it is necessary to discriminate between the
crude deliverance of sense-awareness and our intellectual theories. The
lapse of time is a measurable serial quantity. The whole of scientific
theory depends on this assumption and any theory of time which fails to
provide such a measurable series stands self-condemned as unable to
account for the most salient fact in experience. Our difficulties only
begin when we ask what it is that is measured. It is evidently something
so fundamental in experience that we can hardly stand back from it and
hold it apart so as to view it in its own proportions.

We have first to make up our minds whether time is to be found in nature
or nature is to be found in time. The difficulty of the latter
alternative--namely of making time prior to nature--is that time then
becomes a metaphysical enigma. What sort of entities are its instants
or its periods? The dissociation of time from events d