THE theory of libido which I have advanced ¹ has met with
many misunderstandings and, in some quarters, complete
repudiation; it may therefore not be amiss if I again take up
the fundamental concepts of this theory.

It is a generally recognized truth that physical events can
be looked at in two ways, that is, from the mechanistic and
from the energic standpoint. ² The mechanistic view is purely
causal; from this standpoint an event is conceived as the
result of a cause, in the sense that immutable substances
change their relationships to one another according to
fixed laws.

The energic view-point on the other hand is in essence
final ³ ; the event is traced from effect to cause on the
assumption that energy forms the essential basis of changes
in phenomena, that it maintains itself as a constant throughout
these changes, and finally leads to an entropy, a condition of
general equilibrium. The flow of energy has a definite
direction (goal), in that it follows the fall of potential in a way
that cannot be reversed. The idea of energy is not that of
a substance moved in space; it is a concept abstracted from

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