In the middle ages 213

can give to itself, without some foreign influence

coming from without, this compleme^it of reality,

by virtue of which it passes from one state into

another. Quidquid movetur ah alio movetur. The

principle of contradiction requires this; and the

principle of contradiction, according to which a

thing cannot in the same aspect both be and not be,

is a law of mental life, as well as a law of realitj''.

For, if a thing could change its own state (whether

substantial or accidental) unaided from without, it

would possess before acquiring, — it would alreadj^

be what is not yet. This is of course absurd. The

water is in potency of changing into oxygen; but

without the electric current, — without the interven-

tion of something else — the water could not, by it-

self, give to itself new determinations. This other

thing by which water changes into oxygen and

hydrogen is called the efficient cause.

However, this active cause is itself carried into

the nexus of becoming. The electrical energy could

not appear without undergoing, in its turn, the

action of other efficient causes. The whole process

expands, very much as when a stone is thrown into

still water the waves spread out from the centre,

each acting upon the next in succession. Moreover,

the process becomes complicated, for every action

of a being A on a being B is doubled by a reaction

of B on A. Nature is an inextricable tissue of effi-

cient causes, of becomings, of passages from po-

214 PHILOSOPHY AND CIVILIZATION

tency to act. Newton's law of gravitation, the law

of the equilibrium of forces, the law of the conser-

vation of energy, — these are all so many formulas

which state in precise form the influence of one be-

ing upon another.

But, — and there is of course a but — we cannot

continue the process to infinity. For, in that case,

change would be an illusion, and this would involve

denying the very evidence itself. The initial

motion demands a starting point, an original im-

petus. This absolute beginning is possible only

on the condition that a Being exists who is beyond

all change, — in whom nothing can become, and

who is therefore immutable. That being is God.

Now, God cannot set in motion the series of

changes, constituted of act and potency, except by

an impulse which leaves free and undisturbed His

own impassibility. For, however slight the modi-

fication which one supposes this act (of changing

others) to cause in Him, it would still be a change,

and hence something new and requiring explana-

tion afresh, — by recourse to the intervention of a

still higher being. Thus the process would be end-

less, imless God is the "prime mover unmoved."

Let us suppose that one decides to build a house,

and that one wants it to be suppoi-ted solidly. To

this end he lays deep the foundations which must

support the building. Deep he digs, and still

deeper, and ever deeper, in order to obtain a base