Other points will, however, be made the subjects of incidental discussion. It
will be necessary to premise the following notation.
The universe of conceivable objects is represented by 1 or unity. This I
assume as the primary and subject conception. All subordinate conceptions
of class are understood to be formed from it by limitation, according to the
following scheme.
Suppose that we have the conception of any group of objects consisting of
Xs, Ys, and others, and that \textit{x}, which we shall call an elective symbol, represents
the mental operation of selecting from that group all the Xs which it contains,
or of fixing the attention upon the Xs to the exclusion of all which are not Xs,
\textit{y} the mental operation of selecting the Ys, and so on; then, 1 or the universe
being the subject conception, we shall have
$x1 \text{ or } x = \text{ the class X}$,
$y1 \text{ or } y = \text{ the class Y}$,
$xyl \text{ or } xy = \text{ the class each member of which is both X and Y}$,
