They tell us gravely that there is no effect without a cause; they
repeat to us very often that the world did not create itself. But the
universe is a cause, not an effect; it is not a work, has not been made,
because it was impossible that it should be made. The world has always
been, its existence is necessary. It is the cause of itself. Nature,
whose essence is visibly acting and producing, in order to fulfill her
functions, as we see she does, needs no invisible motor far more unknown
than herself. Matter moves by its own energy, by the necessary result of
its heterogeneity; the diversity of its movements or of its ways of
acting, constitute only the diversity of substances; we distinguish one
being from another but by the diversity of the impressions or movements
which they communicate to our organs.