ensations of vision and touch. Indeed, it will perhaps not be
disputed that the ordinary vident man derives from the sensations of
vision his most common spatial conceptions. We propose, therefore, to
inquire very briefly how the character of spatial extension becomes
associated with the data of Vision.

The objects of Vision appear to be displayed before us in immense
multitude, each distinct from its adjacent neighbour, yet all
inter-related as parts of one single whole--the presentation thus
constituting what is called Extensity.

This is the most commonly employed meaning of the term spatial. Yet it
is evidently in its origin rather temporal than spatial. In ordinary
movement we encounter by touch various obstacles, but only a very few of
these impress us at any one moment of time. On the contrary, they
succeed one after the other. To the blind, therefore, as Platner long
ago remarked: Time serves instead of Space. In Vision, on the other
hand, a large number, which it would take a very long time to encounter
in touch, are presented _simultaneously_. In this there is an immense
practical advantage, the result being that we come habitually to direct
our every action by reference to the data of Sight. Now it is because
these data--so simultaneously presented--are employed by us as the
guides of action that their presentation acquires the character which we
denominate Extensity. The simultaneous occurrence of a large number of
Sounds does not seem to us to present such a character. But let us
suppose that all the objects which constitute obstacles to our Activity
emitted Sounds by which they were recognised; it is not doubtful that
these would then come to be employed by us as the guides of our Activity
and would acquire in our minds the character of Extensity. They would
arrange themselves in a cotemporaneous, extensive, or spatial relation
to one another just as the objects of Vision do at present.

It is only, therefore, when we come to employ the simultaneous
presentation of Vision as the instrument of our Activity and the guide
of Action that it acquires the character commonly called extensive.
_Successive_ visual sensations convey no extensive suggestion.

It is important to realise the nature of this peculiar feature in the
data of Vision. The sounds which we hear, the odours which we smell, are
the immediate result of certain undulations affecting the appropriate
organ of sensation. We refer these to the obj