facts which we shall know bye and bye will belong. Thus, once we have
classified facts as belonging to two classes, daylight and darkness,
and have observed the invariable alternation of facts belonging to
these classes, then, whenever we know directly facts which can be
classed as daylight, we can predict, according to our law of the
alternation of the two classes, that bye and bye these facts will give
place to others which can be classed as darkness and that bye and bye
these in their turn will be replaced by facts which can again be
classed as daylight. The practical value of being able to make even
such elementary predictions as these is obviously enormous, and this
value increases as applied science, which is built up simply by the
formulation of more and more comprehensive general laws of this type,
widens the field of facts which can be explained. Once the laws are
known, moreover, we are able to say to what class the facts must have
belonged which preceded a fact of any given class just as easily as we
can say to what class the facts which are to follow it will belong.
Thus, given a fact which can be classed as daylight, we can infer, by
means of the law of the alternation of the classes daylight and
darkness, not only that facts which can be classed as darkness will
follow bye and bye, but also that facts of that class must have gone
before. In this way we can explain the causes of all classifiable
facts equally with their effects and so bridge over the gaps in our
direct knowledge by creating a unified plan of the interrelations of
all the classes to which facts can belong. By means of this plan we
can explain any fact (that is classify its causes and effects),
provided we can fit it into one or other of the known classes. This
again is of enormous practical use because, when we know to what class
present facts must belong if they are to be followed by the class of
facts which we want, or not to be followed by those which we do not
want, we can arrange our present facts accordingly.