possible, as we shall see more fully in later chapters, that the
sensuous material of perception and imagination and memory
is qualitatively one and the same. Visual mental stuff, for
example, whether perceptually or ideationally produced, is
sui generis, and totally unlike any other kind of mental stuff,
such as auditory or olfactory.

It will be seen that the radical distinction above mentioned
between the perceptual consciousness of objects and such con-
sciousness of them as we may have in memory and imagina-
tion rests upon a physiological basis, i. e., the presence or
absence of sense organ activity. The only difference on the
mental side is commonly to be found in the intensity and
objectivity of the two. Perceptions are ordinarily more
intense, and feel more as though given to us, than do our
memories or imaginings. Nevertheless, there are many per-
sons whose imagery frequently takes on an almost perceptual
vividness and is followed by motor consequences such as nor-
mally belong to sense stimuli. The thought of blood, for
example, or the description of a wound will in these cases
elicit the most life-like visual images followed by nausea and
even vomiting. In hallucination, too, it seems as though mere
mental images assumed the vividness and externality of per-
cepts; and in the case of very faint stimulations, e. g., of
sound or colour, we cannot always be confident whether we
have really perceived something, or merely imagined it. This
principle of distinguishing the two is, therefore, not always
to be depended upon. Fortunately for our practical interests,
the distinction is generally valid and we do not often confuse
what we really perceive, with what we imagine.

It must be said that certain distinguished psychologisits
maintain that there is a real difference in quality between
perceptual and ideational material. They base their view
partly upon introspective grounds and partly upon alleged evi-
dence from cases of mental disease, where the power to obtain
images is lost without entire loss of perceptual capacity. To

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