qualities with which it is united, does by that means frame to itself
abstract ideas. For example, there is perceived by sight an object
extended, coloured, and moved: this mixed or compound idea the mind
resolving into its simple, constituent parts, and viewing each by itself,
exclusive of the rest, does frame the abstract ideas of extension, colour,
and motion. Not that it is possible for colour or motion to exist without
extension; but only that the mind can frame to itself by ABSTRACTION the
idea of colour exclusive of extension, and of motion exclusive of both
colour and extension.

8. OF GENERALIZING [Note].--Again, the mind having observed that in the
particular extensions perceived by sense there is something COMMON and
alike IN ALL, and some other things peculiar, as this or that figure or
magnitude, which distinguish them one from another; it considers apart or
singles out by itself that which is common, making thereof a most abstract
idea of extension, which is neither line, surface, nor solid, nor has any
figure or magnitude, but is an idea entirely prescinded from all these. So
likewise the mind, by leaving out of the particular colours perceived by
sense that which distinguishes them one from another, and retaining that
only which is COMMON TO ALL, makes an idea of colour in abstract which is
neither red, nor blue, nor white, nor any other determinate colour. And,
in like manner, by considering motion abstractedly not only from the body
moved, but likewise from the figure it describes, and all particular
directions and velocities, the abstract idea of motion is framed; which
equally corresponds to all particular motions whatsoever that may be
perceived by sense.

[Note: Vide Reid, on the Intellectual Powers of Man, Essay V,
chap iii. sec. 1, edit. 1843]