These two terms are often confused in the public mind. Psychologically,
however, they are quite distinct. The consciousness of material objects
through the senses is called “perception.” We _perceive_ them. On the
other hand, we _conceive_ an idea. Perceptions depend upon our senses
and brain-processes; conceptions may be independent of the former. It
is true, however, that perceptions depend upon the inner workings of
the mind, as well as upon sense-impressions.
When we perceive a thing, we recognize or “know” that thing. But we
do not yet think _about_ it, associate it with other things, etc.
This higher process of the mind has been termed “apperception.” It is
applied to the process by which the mind goes out to meet the incoming
perception, and elaborates it by higher processes of association, etc.
Thus, through the sensation of sight, you perceive an orange. But you
do not yet say to yourself: “This is a nice, juicy orange; I like
oranges; it will taste sweet; it will quench my thirst,” etc. These
associative processes of the mind may be classed, roughly, under the
process of “apperception.” They represent syntheses and associations.
We are all subject to fallacies of perception. Our senses may deceive
us; or we may draw wrong deductions from genuine sense impressions.
There are thus illusions and hallucinations, which resemble one
another, but which may be distinguished thus: _Illusions_ are caused
by wrong inferences from actual, objective sense impressions--as, for
instance when a hat and coat hanging on the wall are mistaken for a man
standing at the foot of the bed, etc. In the case of a _hallucination_,
however, there is no physical substratum of reality in the outer
world; the whole thing is created from within, whence it is projected
outward,--into space. Some hallucinations of this type seem as “real”
as a solid object would be.
Hallucinations often result from diseased or irritated sense-organs,
the sense-centers in the brain, etc. The condition of the blood
may have much to do with this. Delirium tremens is a good example
of hallucination of this type. But not all hallucinations are
abnormal; we all have them at night when we fall asleep and dream.
We then experience hallucinatory pictures of all sorts of things
which are not really “there.” There seem to be, also, odd cases of
“telepathically induced hallucinations,” as I have described in my
booklet on “Psychical Research,” in this series. In such cases, the
subject appears to be perfectly normal at the time. There can be no
doubt, however, that most hallucinations denote some form of mental or
physical disease--as the hallucinations of the insane, of drug fiends,
etc., amply testify.