r we dig beneath them, the more we have to do with a reality which
changes as a whole in its inmost states, as if an accumulative memory of
the past made it impossible to go back again. The mechanistic instinct
of the mind is stronger than reason, stronger than immediate experience.
The metaphysician that we each carry unconsciously within us, and the
presence of which is explained, as we shall see later on, by the very
place that man occupies amongst the living beings, has its fixed
requirements, its ready-made explanations, its irreducible propositions:
all unite in denying concrete duration. Change _must_ be reducible to an
arrangement or rearrangement of parts; the irreversibility of time
_must_ be an appearance relative to our ignorance; the impossibility of
turning back _must_ be only the inability of man to put things in place
again. So growing old can be nothing more than the gradual gain or loss
of certain substances, perhaps both together. Time is assumed to have
just as much reality for a living being as for an hour-glass, in which
the top part empties while the lower fills, and all goes where it was
before when you turn the glass upside down.

True, biologists are not agreed on what is gained and what is lost
between the day of birth and the day of death. There are those who hold
to the continual growth in the volume of protoplasm from the birth of
the cell right on to its death.[5] More probable and more profound is
the theory according to which the diminution bears on the quantity of
nutritive substance contained in that "inner environment" in which the
organism is being renewed, and the increase on the quantity of unexcreted
residual substances which, accumulating in the body, finally "crust it
over."[6] Must we however--with an eminent bacteriologist--declare any
explanation of growing old insufficient that does not take account of
phagocytosis?[7] We do not feel qualified to settle the question. But
the fact that the two theories agree in affirming the constant accumulation
or loss of a certain kind of matter, even though they have little in common
as to what is gained and lost, shows pretty well that the frame of the
explanation has been furnished _a priori_. We shall see this more and more
as we proceed with our study: it is not easy, in thinking of time, to
escape the image of the hour-glass.

The cause of growing old must lie deeper. We hold that there is unbroken
continuity between the evolution of