Of course, not all scientific concepts get reduced to purely
physical characteristics. But in the natural sciences, and
especially biology, concepts that cannot be recast in this
way often fail to develop much traction. One reason is that,
to get a lot of use out of a concept, we need to be able to
recognize quickly and easily what it refers to (we cannot
afford to set up crosses every time we want to call
something a gene). There is an even more compelling
reason: history tells us that it is often only in the act of
searching for the physical equivalents of abstract notions
that we tend to learn whether those abstractions refer to
anything real at all. As a case in point, consider ‘phlogiston’,
an idea introduced in the seventeenth century to explain
the process of combustion. The concept of phlogiston
admits precise operational definition - it is the substance
universally removed from all materials upon burning - but
it happens that no substance with definite physical characteristics
has ever been found that satisfies this operational
definition. Indeed, it was ultimately the discovery that one
of those physical characteristics would need to be the
unlikely property of ‘negative mass’, which consigned
phlogiston to the conceptual discard pile.