s"; "Mary loves dolls less than Lucy does"; "A
had greater devotion to his country than B had"; are reports of
quantitative differences, of differences in the amount of what is
assumed to be the same kind of thing. A qualitative difference exists
when some quality or trait possessed by one individual is lacking in the
other. Thus, "Tom knows German, Dick does not"; "A is artistic, B is
scientific"; "C is a man of thought, D is a man of action"; are reports
of the fact that Tom has some positive amount or degree of the trait
"knowledge of German" while Dick has none of it; that A has some
positive amount of ability and interest in art while B has zero; whereas
B has a positive amount of ability in science, of which A has none; and
so on.

A qualitative difference in intellect or character is thus really a
quantitative difference wherein one term is zero, or a compound of two
or more quantitative differences. All intelligible differences are
ultimately quantitative. The difference between any two individuals, if
describable at all, is described by comparing the amounts which A
possesses of various traits with the amounts which B possesses of the
same traits. In intellect and character, differences of kind between one
individual and another turn out to be definable, if defined at all, as
compound differences of degree.

If we could list all the traits, each representing some one
characteristic of human nature, and measure the amount of each of them
possessed by a man, we could represent his nature--read his
character--in a great equation. John Smith would equal so many units of
this, plus so many units of that, and so on. Such a mental inventory
would express his individuality conceivably in its entirety and with
great exactitude. No such list has been made for any man, much less have
the exact amounts of each trait possessed by him been measured. But in
certain of the traits, many individuals have been measured; and certain
individuals have been measured, each in a large number of traits.

It is useless to recount the traits in which men have been found to
differ. For there is no trait in which they do not differ. Of course, if
the scale by which individuals are measured is very coarsely divided,
their differences may be hidden. If, for example, ability to learn is
measured on a scale with only two divisions, (1) "ability to learn less
than the average kitten can" and (2) "ability to learn more than the
average kitten