connected with variations of sensibility.

Our scheme of sense-illusions is now complete. First of all, we shall
take up the passive illusions, beginning with those which are
conditioned by special circumstances in the organism. After that we
shall illustrate those which depend on peculiar circumstances in the
environment. And finally, we shall separately consider what I have
called the active illusions of sense.

It is to be observed that these illusions of perception properly so
called, namely, the errors arising from a wrong interpretation of an
impression, and, not from a confusion of one impression with another are
chiefly illustrated in the region of the two higher senses, sight and
hearing. For it is here, as we have seen, that the interpretative
imagination has most work to do in evolving complete percepts of
material, tangible objects, having certain relations in space, out of a
limited and homogeneous class of sensations, namely, those of light and
colour, and of sound. As I have before observed, tactual perception, in
so far as it is the recognition of an object of a certain size,
hardness, and distance from our body, involves the least degree of
interpretation, and so offers little room for error; it is only when
tactual perception amounts to the _recognition_ of an individual object,
clothed with secondary as well as primary qualities, that an opening for
palpable error occurs.

With respect, however, to the first sub-class of these illusions,
namely, those arising from organic peculiarities which give a twist, so
to speak, to the sensation, no very marked contrast between the
different senses presents itself. So that in illustrating this group we
shall be pretty equally concerned with the various modes of perception