l not
be attended to, because they are not what we are busied with. We
shall be _passive_ towards the white sensations while we are
_active_ towards the black and red ones; we shall not measure the
white; not sweep our glance along it as we do along the red and the
black. And as _ceteris paribus_ our tense awareness of active states
always throws into insignificance a passive state sandwiched
between them; so, bent as we are upon our red and black extensions,
and their comparative lengths and directions, we shall treat the
uninteresting white extensions as a _blank,_ a gap, as that which
separates the objects of our active interest, and takes what existence
it has for our mind only from its relation of separating those
interesting actively measured and compared lines. Thus the
difference between our _active perception_ and our merely _passive
sensation_ accounts for the fact that every visible shape is composed
of lines (or bands) measured and compared with reference to our
own ocular adjustments and our axis and centre; lines existing, as
we express it, in _blank space,_ that is to say space not similarly
measured; lines, moreover, _enclosing_ between each other more of
this blank space, which is not measured in itself but subjected to the
measurement of its enclosing lines. And similarly, every _audible_
Shape consists not merely of sounds enclosing _silence,_ but of
heard tones between which we are aware of the intervening _blank
interval_ which _might have been_ occupied by the intermediary
tones and semitones. In other words, visible and audible Shape is
composed of alternations between _active,_ that is _moving,_
measuring, referring, comparing, attention; and _passive,_ that is
comparatively sluggish _reception_ of mere sensation.

This fact implies another and very important one, which I have
indeed already hinted at. If perceiving shape means comparing lines
(they may _be bands,_ but we will call them _lines),_ and the lines
are measured only by consecutive eye movements, then the act of
comparison evidently includes the co-operation, however
infinitesimally brief, of _memory._ The two halves of this
Chippendale chair-back exist simultaneously in front of my eyes,
but I cannot take stock simultaneously of the lengths and orientation
of the curves to the right and the curves of the left. I must hold over
the image of one half, and unite it, somewhere in what we call "the
mind"--with the other; nay, I must do