t of
imagination, the less the additional force of external stimulus required
to excite them to full activity.

Considering the first division, passive illusions, a little further, we
shall see that they may be broken up into two sub-classes, according to
the causes of the errors. In a general way we assume that the impression
always answers to some quality of the object which is perceived, and
varies with this; that, for example, our sensation of colour invariably
represents the quality of external colour which we attribute to the
object. Or, to express it physically, we assume that the external force
acting on the sense-organ invariably produces the same effect, and that
the effect always varies with the external cause. But this assumption,
though true in the main, is not perfectly correct. It supposes that the
organic conditions are constant, and that the organic process faithfully
reflects the external operation. Neither of these suppositions is
strictly true. Although in general we may abstract from the organism and
view the relation between the external fact and the mental impression as
direct, we cannot always do so.

This being so, it is possible for errors of perception to arise through
peculiarities of the nervous organization itself. Thus, as I have just
observed, sensibility has its limits, and these limits are the
starting-point in a certain class of widely shared or _common_
illusions. An example of this variety is the taking of the two points of
a pair of compasses for one by the hand, already referred to. Again, the
condition of the nervous structures varies indefinitely, so that one and
the same stimulus may, in the case of two individuals, or of the same
individual at different times, produce widely unlike modes of sensation.
Such variations are clearly fitted to lead to gross _individual_ errors
as to the external cause of the sensation. Of this sort is the illusory
sense of temperature which we often experience through a special state
of the organ employed.

While there are these errors of interpretation due to some peculiarity
of the organization, there are others which involve no such peculiarity,
but arise through the special character or exceptional conformation of
the environment at the moment. Of this order are the illusions connected
with the reflection of light and sound. We may, perhaps, distinguish the
first sub-class as organically conditioned illusions, and the second as
extra-organica