cal necessity. Neither formal nor final causes exist, but only
efficient causes. All that happens takes its origin in the activity of an
external cause, and not in itself; a body at rest (or in motion) remains
at rest (or in motion) forever, unless affected by another in a contrary
sense. And as bodies and their changes constitute the only objects of
philosophy, so the mathematical method is the only correct method.

There are two kinds of bodies: natural bodies, which man finds in nature,
and artificial bodies, which he himself produces. By the latter Hobbes
refers especially to the state as a human artefact. Man stands between the
two as the most perfect natural body and an element in the political body.
Philosophy, therefore, besides the introductory _philosophia prima_, which
discusses the underlying concepts, consists of three parts: physics,
anthropology, and politics. Even the theory of the state is capable of
demonstrative treatment; moral phenomena are as subject to the law of
mechanical causation as physical phenomena.

The first factor in the cognitive process is an impression on a
sense-organ, which, occasioned by external motion, continues onward to the
heart and from this center gives rise to a reaction. The perception or
sensation which thus arises is entirely subjective, a function of the
knower merely, and in no way a copy of the external movement. The
properties light, color, and sound, which we believe to be without us, are
merely internal phenomena dependent on outer and inner motions, but with no
resemblance to them. Memory consists in the lingering effects or residuary
traces of perception; it is a sense or consciousness of having felt before
_(sentire se sensisse meminisse est_), and ideas are distinguished from
sensations as the perfect from the present tense. Experience is the
totality of perceptions retained in memory, together with a certain
foresight of the future after the analogy of the past. These stages of
cognition, which can yield prudence but not necessary and universal
knowledge, are present in animals as well as men. The human capacity for
science is dependent on the faculty of speech; words are conventional
signs to facilitate the retention and communication of ideas. As the
memory-images denoted by words are weaker, fainter, and less clearly
discriminated than the original sensations, it comes to pass that a number
of similar ideas of memory receive a common name. Thus abstract gene