to economics and sociology, a gradation whose correlations and
implications have not yet received adequate recognition, and which does
profoundly affect the method of study and research in each science.

Let me repeat in slightly altered terms some of the points raised in the
preceding sections. I have doubted and denied that there are identically
similar objective experiences; I consider all objective beings as
individual and unique. It is now understood that conceivably only in the
subjective world, and in theory and the imagination, do we deal with
identically similar units, and with absolutely commensurable quantities.
In the real world it is reasonable to suppose we deal at most with
PRACTICALLY similar units and PRACTICALLY commensurable quantities. But
there is a strong bias, a sort of labour-saving bias, in the normal
human mind, to ignore this, and not only to speak but to think of a
thousand bricks or a thousand sheep or a thousand Chinamen as though
they were all absolutely true to sample. If it is brought before a
thinker for a moment that in any special case this is not so, he slips
back to the old attitude as soon as his attention is withdrawn. This
type of error has, for instance, caught many of the race of chemists,
and ATOMS and IONS and so forth of the same species are tacitly assumed
to be similar to one another.

Be it noted that, so far as the practical results of chemistry and
physics go, it scarcely matters which assumption we adopt, the number of
units is so great, the individual difference so drowned and lost. For
purposes of enquiry and discussion the incorrect one is infinitely more
convenient.

But this ceases to be true directly we emerge from the region of
chemistry and physics. In the biological sciences of the eighteenth