laws of
sensibility, and how much must be put down to the reaction of the mind
in the shape of attention and discrimination? For our present purpose we
may say that, whenever a deliberate effort of attention does not suffice
to alter the character of a sensation, this may be pretty safely
regarded as a net result of the nervous process, and any error arising
may be referred to the later stages of the process of perception. Thus,
for example, the taking of the two points of a pair of compasses for
one, where the closest attention does not discover the error, is best
regarded as arising, not from a confusion of the sense-impression, but
from a wrong interpretation of a sensation, occasioned by an
overlooking of the limits of local discriminative sensibility.

_Misinterpretation of the Sense-Impression._

Enough has been said, perhaps, about those errors of perception which
have their root in the initial process of sensation. We may now pass to
the far more important class of illusions which are related to the later
stages of perception, that is to say, the process of interpreting the
sense-impression. Speaking generally, one may describe an illusion of
perception as a misinterpretation. The wrong kind of interpretative
mental image gets combined with the impression, or, if with Helmholtz we
regard perception as a process of "unconscious inference," we may say
that these illusions involve an unconscious fallacious conclusion. Or,
looking at the physical side of the operation, it may be said that the
central course taken by the nervous process does not correspond to the
external relations of the moment.

As soon as we inspect these illusions of interpretation, we see that
they fall into two divisions, according as they are connected with the
process of _suggestion_, that is to say, the formation of the
interpretative image so far as determined by links of association with
the actual impression, or with an independent process of _preperception_
as explained above. Thus, for example, we fall into the illusion of
hearing two voices when our shout is echoed back, just because the
second auditory impression irresistibly calls up the image of a second
shouter. On the other hand, a man experiences the illusion of seeing
spectres of familiar objects just after exciting his imagination over a
ghost-story, because the mind is strongly predisposed to frame this kind
of percept. The first class of illusions arises from without, th