When a baby first begins to notice objects about it, everything is
doubtless more or less vague and confused. Gradually, order appears out
of chaos. Normal adults have a fairly clear idea of the sizes, shapes
and distances of objects--within certain limits. How are we enabled to
do this?
An enormous literature exists upon this subject. Only a few essentials
can be given here. First of all, it is probably true that the general
feeling of size exists as a definite quality in sensation--of intensity
of voluminousness. Varied sensations are inwardly compared with one
another, and, so to say, checked-off. Smaller objects are discriminated
within the object looked at; the object is usually seen in relation
to other objects, etc. James holds that “extensity, discernible in
each and every sensation ... is the original sensation of space,
out of which all the exact knowledge about space that we afterwards
come to have is woven by processes of discrimination, association
and selection.” These later powers are developed as the mind itself
develops; on the other hand, sensations with a certain degree of innate
extensity in them are postulated--and are denied by others!
Ouspensky (“Tertium Organum”) asserts that man is the only
three-dimensional animal; the lower animals are naturally
two-dimensional, and that what we perceive as the third dimension they
perceive as movement. They see only flat surfaces. This is, however, a
much-disputed point, into which we have not time to enter now; it also
takes us into the realm of animal psychology.