Posterior Analytics   

from an event which occurred in the past that a future event will
occur. The reason of this is that the middle must be homogeneous, past
when the extremes are past, future when they are future, coming to
be when they are coming-to-be, actually existent when they are
actually existent; and there cannot be a middle term homogeneous
with extremes respectively past and future. And it is a further
difficulty in this theory that the time interval can be neither
indefinite nor definite, since during it the inference will be
false. We have also to inquire what it is that holds events together
so that the coming-to-be now occurring in actual things follows upon a
past event. It is evident, we may suggest, that a past event and a
present process cannot be 'contiguous', for not even two past events
can be 'contiguous'. For past events are limits and atomic; so just as
points are not 'contiguous' neither are past events, since both are
indivisible. For the same reason a past event and a present process
cannot be 'contiguous', for the process is divisible, the event
indivisible. Thus the relation of present process to past event is
analogous to that of line to point, since a process contains an
infinity of past events. These questions, however, must receive a more
explicit treatment in our general theory of change.
The following must suffice as an account of the manner in which
the middle would be identical with the cause on the supposition that
coming-to-be is a series of consecutive events: for in the terms of
such a series too the middle and major terms must form an immediate
premiss; e.g. we argue that, since C has occurred, therefore A
occurred: and C's occurrence was posterior, A's prior; but C is the
source of the inference because it is nearer to the present moment,
and the starting-point of time is the present. We next argue that,
since D has occurred, therefore C occurred. Then we conclude that,
since D has occurred, therefore A must have occurred; and the cause is
C, for since D has occurred C must have occurred, and since C has
occurred A must previously have occurred.
If we get our middle term in this way, will the series terminate
in an immediate premiss, or since, as we said, no two events are
'contiguous', will a fresh middle term always intervene because
there is an infinity of middles? No: though no two events are
'contiguous', yet we must start from a premiss consisting of a
middle and the present event as major. The like is true of future
events too, since if it is true to say that D will exist, it must be a
prior truth to say that A will exist, and the cause of this conclusion
is C; for if D will exist, C will exist prior to D, and if C will
exist, A will exist prior to it. And here too the same infinite
divisibility might be urged, since future events are not 'contiguous'.
But here too an immediate basic premiss must be assumed. And in the
world of fact this is so: if a house has been built, then blocks
must have been quarried and shaped. The reason is that a house
having been built necessitates a foundation having been laid, and if a
foundation has been laid blocks must have been shaped beforehand.
Again, if a house will be built, blocks will similarly be shaped
beforehand; and proof is through the middle in the same way, for the
foundation will exist before the house.
Now we observe in Nature a certain kind of circular process of
coming-to-be; and this is possible only if the middle and extreme
terms are reciprocal, since conversion is conditioned by reciprocity
in the terms of the proof. This-the convertibility of conclusions
and premisses-has been proved in our early chapters, and the
circular process is an instance of this. In actual fact it is
exemplified thus: when the earth had been moistened an exhalation
was bound to rise, and when an exhalation had risen cloud was bound to
form, and from the formation of cloud rain necessarily resulted and by
the fall of rain the earth was necessarily moistened: but this was the
starting-point, so that a circle is completed; for posit any one of
the terms and another follows from it, and from that another, and from
that again the first.