He is, therefore, the efficient cause of the statue. The
formal cause Aristotle defines as the substance and essence of the
thing. Now the essence of a thing is given in its definition. But the
definition is the explication of the concept. Therefore the formal
cause is the concept, or, as Plato would call it, the Idea of the
thing. Plato's Ideas thus reappear in Aristotle as formal causes. The
final cause is the end, purpose, or aim, towards which the movement is
directed. When a statue is being produced, the end of this activity,
what the sculptor aims at, is the completed statue itself. And the
final cause of a thing in general is the thing itself, the completed
being of the object.

We can see at once how much wider this conception of causation is than
the modern conception. If we take Mill's definition of a cause as the
best expression of modern scientific ideas, we find that he defines a
cause as the "invariable and unconditional antecedent of a
phenomenon." This cuts out final causes at once. For {270} the final
cause is the end, and is not an antecedent in time. It also does not
include formal causes. For we do not now think of the concept of a
thing as being part of its cause. This leaves us with only material
and efficient causes, and these correspond roughly to the modern
notions of matter and energy. Even the efficient causes of Aristotle,
however, appear on further consideration, to be excluded from the
modern idea of causation. For, though the efficient cause is the
energy which produces motion, modern science regards it as purely
mechanical energy, whereas Aristotle thinks of it, as we shall see, as
an ideal force, operating not from the beginning but from the end. But
it must not be supposed that, in saying that the modern idea of
causation excludes formal and final causes, we mean that Aristotle is
wrong in adding them, or that the modern idea is better than
Aristotle's. It is not a question of better and worse at all. Modern
science does not in any way deny the reality of formal and final
causes. It merely considers them to be outside its sphere. It is no
business of science whether they exist or not. As knowledge advances,
differentiation and division of labour occur. Science takes as its
province mechanical causes, and leaves formal and final causes to the
philosopher to explicate. Thus, for example, formal causes are not
considered by science because they are not, in the modern sense,
causes at all