hat the
drug did cure her child, this knowledge by itself would have been
practically valueless, because the only legitimate inference would
have been that an exactly similar dose would have the same effect
in exactly similar circumstances. But, as we shall find, though
practically valueless, a single ascertained causal sequence is of
supreme value in testing scientific speculations as to the underlying
causes.

(2) We have next to see whether there are any other rational
expectations based on observed facts. We may lay down as a principle
the following:--

_If a conjunction or coincidence has constantly been repeated within
our experience, we expect it to recur and believe that it has recurred
outside our experience._

How far such expectations are rational, and with what degrees of
confidence they should be entertained, are the questions for the Logic
of Inference, but we may first note that we do as a matter of habit
found expectations on repeated coincidence, and indeed guide our
daily life in this way. If we meet a man repeatedly in the street at
a certain hour, we go out expecting to meet him: it is a shock to our
expectations, a surprise, when we do not. If we are walking along a
road and find poles set up at regular intervals, we continue our walk
expecting to find a pole coincident with the end of each interval.

What Mill calls the uniformities of Nature, the uniformities expressed
in general propositions, are from the point of view of the observer,
examples of repeated coincidence. Birth, growth, decay, death, are not
isolated or variable coincidences with organised being: all are born,
all grow, all decay, and all die. These uniformities constitute
the order of Nature: the coincidences observed are not occasional,
occurring once in a way or only now and then; they turn up again and
again. Trees are among the uniformities on the varied face of Nature:
certain relations between the soil and the plant, between trunk,
branches, and leaves are common to them. For us who observe, each
particular tree that comes under our observation is a repetition of
the coincidence. And so with animals: in each we find certain tissues,
certain organs, conjoined on an invariable plan.

Technically these uniformities have been divided into uniformities
of Sequence and uniformities of Coexistence. Thus the repeated
alternation of day and night is a uniformity of Sequence: the
invariable conjunction of inertia with weight is