hat my idea of causation as a
principle in the external world is derived from my knowledge of this
principle in the internal world. For I find that my idea of force and
energy in the external world is a mere projection of the idea which I
have of effort within the region of my own consciousness; and therefore
my only idea of causation is that which is originally derived from the
experience which I have of this principle as obtaining among my own
mental modifications.

If once we see plainly that the idea of causation is derived from
within, and that what we call the evidence of physical causation is
really the evidence of mental modifications following one another in a
definite sequence, we shall then clearly see, not merely that we have no
evidence, but that we _can have_ no evidence of causation as proceeding
from object to subject. However cogent the evidence may appear at first
sight to be, it is found to vanish like a cloud as soon as it is exposed
to the light of adequate contemplation. In the very act of thinking the
evidence, we are virtually denying its possibility as evidence; for as
evidence it appeals only to the mind, and since the mind can only know
its own sequences, the evidence must be presenting to the mind an
account of its own modifications; from the mere fact, therefore, of its
being accepted as thinkable, the evidence is proved to be illusory.

To uneducated men it appears an indisputable fact of 'common sense' that
the colour of a flower exists as perceived in the flower, apart from any
relation to the percipient mind. A physiologist has gone further into
the thicket of things, and finds that the way is not so simple as this.
He regards the quality of colour as necessarily related to the faculty
of visual perception; does not suppose that the colour exists _as such_
in the flower, but thinks of the something there as a certain order of
vibrations which, when brought into relation with consciousness through
the medium of certain nerves, gives rise to the perception experienced;
and in order to account for the translation into visual feeling of an
event so unlike that feeling as is the process taking place in the
flower, physiologists have recourse to an elaborate theory, such as that
of Helmholtz or Hering. In other words, physiologists here fully
recognize that colour, or any other thing perceived, only exists _as
perceived_ in virtue of a subjective element blending with an objective;
the thing