a class or word or other symbol. All description and explanation of
facts consists in substitutions of this kind. The explanation applies
provided the abstraction is based on fact, that is, provided it is
possible to fit the fact to which the explanation is intended to apply
into the class employed to explain it: the general law, for instance,
about the alternation of the classes daylight and darkness will
explain any facts which can be fitted into one or other of these
classes, or again general laws about dogs, such as "dogs lick" will
apply to whatever fact belongs at once to all the simpler classes,
"warm," "rough," "of a certain size, and smell," out of which the
class "dog" is constructed. The general law itself, however, does not
consist of such facts but of abstractions substituted for the facts
themselves. Such substitution is extremely useful and perfectly
legitimate so long as we keep firm hold of the fact as well, and are
quite clear about what is fact and what only symbol. The danger is,
however, that, being preoccupied with describing and explaining and
having used abstractions so successfully for these purposes, we may
come to lose our sense of fact altogether and fail to distinguish
between actual facts and the symbols which we use to explain them.

This, indeed, is just what Bergson thinks really does happen. No doubt
an intelligent physicist is perfectly aware that the vibrations and
wave lengths and electrons and forces by which he explains the changes
that take place in the material world are fictions, and does not
confuse them with the actual facts in which his actual knowledge of
the material world consists. But it is much more doubtful whether he
distinguishes between these actual facts and the common sense material
objects, such as lumps of lead, pieces of wood, and so on, which he
probably believes he knows directly but which are really only
abstractions derived from the facts in order to explain them just as