Incipe, parve puer, risu cognoscere matrem,

he is exhorting the beatific infant to the exercise of the faculty of
comparison.

44. That a common term implies comparison does not need to be
insisted upon. It is because things resemble each other in certain of
their attributes that we call them by a common name, and this
resemblance could not be ascertained except by comparison, at some
time and by some one. Thus a common term, or concept, is the
compressed result of an indefinite number of comparisons, which lie
wrapped up in it like so many fossils, witnessing to prior ages of
thought.

45. In the next product of thought, namely, the proposition, we have
the result of a single act of comparison between two terms; and this
is why the proposition is called the unit of thought, as being the
simplest and most direct result of comparison.

46. In the third product of thought, namely, the inference, we have
a comparison of propositions either directly or by means of a
third. This will be explained later on. For the present we return to
the first product of thought.

47. The nature of singular terms has not given rise to much dispute;
but the nature of common terms has been the great battle-ground of
logicians. What corresponds to a singular term is easy to determine,
for the thing of which it is a name is there to point to: but the
meaning of a common term, like 'man' or 'horse,' is not so obvious as