conception {268} than what is understood by that term in
modern times. I have in previous lectures attempted to make clear the
distinction between causes and reasons. The cause of a thing does not
give any reason for it, and therefore does not explain it. The cause
is merely the mechanism by which a reason produces its consequence.
Death is caused by accident or disease, but these causes explain
nothing as to why death should be in the world at all. Now if we
accept this distinction, we may say that Aristotle's conception of
causation includes both what we have called causes and reasons.
Whatever is necessary, whether facts or principles, whether causes or
reasons, fully to understand the existence of a thing, or the
happening of an event, is included in the Aristotelian notion of
causation.

Taking causation in this wide sense, Aristotle finds that there are
four kinds of causes, the material, the efficient, the formal, and the
final cause. These are not alternative causes; it is not meant that,
to explain anything, one or other of the four must be present. In
every case of the existence or production of a thing all four causes
operate simultaneously. Moreover the same four causes are to be found
both in human and in cosmic production, in the making of manufactured
articles by man and in the production of things by nature. They are
more clearly and easily seen, however, in human production, from which
sphere, therefore, we select our example. The material cause of a
thing is the matter of which it is composed. It is the raw material
which becomes the thing. For example, in the making of a bronze statue
of Hermes, the bronze is the material cause of the statue. This
example might lead one to suppose {269} that Aristotle means by
material cause what we call matter, physical substance, such as brass,
iron, or wood. As we shall see later, this is not necessarily the
case, though it is so in the present instance. The efficient cause is
always defined by Aristotle as the cause of motion. It is the energy
or moving force required to bring about change. It must be remembered
that by motion Aristotle means not merely change of place but change
of any sort. The alteration of a leaf from green to yellow is just as
much motion, in his sense, as the falling of a stone. The efficient
cause, then, is the cause of all change. In the example taken, what
causes the bronze to become a statue, what produces this change, is
the sculptor.