blue object. Nor can
we explain what heat is like, or the smell of tobacco, to those who have
never experienced them; nor the sound of C 128 to one who knows nothing
of the musical scale.

If we distinguish the property of an object from the sensation it
excites in us, we may define any simple property as 'the power of
producing the sensation'; the colour of a flower as the power of
exciting the sensation of colour in us. Still, this gives no information
to the blind nor to the colour-blind. Abstract names may be defined by
defining the corresponding concrete: the definition of 'human nature' is
the same as of 'man.' But if the corresponding concrete be a simple
sensation (as blue), this being indefinable, the abstract (blueness) is
also indefinable.

(b) The second limit of Definition is the impossibility of exhausting
infinity, which would be necessary in order to convey the meaning of the
name of any individual thing or person. For, as we saw in chap. iv., if
in attempting to define a proper name we stop short of infinity, our
list of qualities or properties may possibly be found in two
individuals, and then it becomes the definition of a class-name or
general name, however small the actual class. Hence we can only give a
Description of that which a proper name denotes, enumerating enough of
its properties to distinguish it from everything else as far as our
knowledge goes.

Sec. 8. The five Predicables (Species, Genus, Difference, Proprium,
Accident) may best be discussed in connection with Classification and
Definition; and in giving an account of Classification, most of what has
to be said about them has been anticipated. Their name, indeed, connects
them with the doctrine of Propositions; for Predicables are terms that
may be predicated, classified according to their connotative relation to
the subject of a proposition (that is, according to the relation in
which their connotation stands to the connotation of the subject):
nevertheless, the significance of the relations of such predicates to a
subject is derivative from the general doctrine of classification.

For example, in the proposition 'X is Y,' Y must be one of the five
sorts of predicables in relation to X; but of what sort, depends upon
what X (the subject) is, or means. The subject of the proposition must
be either a definition, or a general connotative name, or a singular
name.

If X be a definition, Y must be a species; for nothing but a genera