sometimes agreeable and beneficial, at others prejudicial and
disagreeable. It is Nature, that in giving him feeling, in supplying him
with sentiment, has endowed him with capacity to choose, the means to
elect those objects, to take those methods that are most conducive, most
suitable, most natural, to his conservation. It is Nature, who when he
has run his race, when he has finished his career, when he has described
the circle marked out for him, conducts him in his turn to his
destruction; dissolves the union of his elementary particles, and
obliges him to undergo the constant, the universal law; from the
operation of which nothing is exempted. It is thus, motion places man in
the matrix of his mother; brings him forth out of her womb; sustains him
for a season; at length destroys him; obliges him to return into the
bosom of Nature; who speedily reproduces him, scattered under an
infinity of forms; in which each of his particles run over again, in the
same manner, the different stages, as necessary as the whole had before
run over those of his preceding existence.

The beings of the human species, as well as all other beings, are
susceptible of two sorts of motion: the one, that of the mass, by which
an entire body, or some of its parts, are visibly transferred from one
place to another; the other, internal and concealed, of some of which
man is sensible, while some takes place without his knowledge, and is
not even to be guessed at, but by the effect it outwardly produces. In a
machine so extremely complex as man, formed by the combination of such a
multiplicity of matter, so diversified in its properties, so different
in its proportions, so varied in its modes of action, the motion
necessarily becomes of the most complicated kind; its dullness, as well
as its rapidity, frequently escapes the observation of those themselves,
in whom it takes place.