are asked to solve in regard to Mathematics does not present itself in
Physics. I am constrained to believe that two and two are four and not
five; I am not constrained to believe that if one event is followed by
another a great many times it will be so followed always. And the
question is, why, without any constraint, I nevertheless so far believe
it that I require special evidence in any given case to convince me to
the contrary. And Kant's answer is irrelevant. He says that we cannot
think the sequence of events unless they fall under the postulates of
thinking, that is, the postulates of science; but this is no answer to
the question. Why do we believe that, unless the contrary be proved,
everything that is observed by the senses is capable of being reduced
under these postulates of thinking? The sequence of things cannot
otherwise be explained; but why should the sequence of all things that
happen be capable of being explained? The question therefore still
remains unanswered. What right have we to assume this Uniformity in
Nature? or, in other words, what right have we to assume that all
phenomena in Nature, observed by our senses, are capable of being
brought within the domain of Science? And to answer this question we
must approach it from a different side.

And there is the more reason for this because it is undeniable that both
the definition and the universality of the relation of cause and effect,
as they were accepted by Hume and his followers, are not accepted by men
in general. In ordinary language something more is meant by cause and
effect than invariable sequence, and the common assumption is not that
all Nature obeys this rule with absolutely no variation, but that the
rule is sufficiently general for all practical purposes.

If then we begin by asking what is the process of Science in dealing
with all questions of causation, we find that this process when reduced
to its simplest elements always consists in referring every event as an
effect to some cause which we know or believe to have produced some
other and similar event. Newton is struck by a falling apple. His first
thought is, 'how hard the blow.' His second is wonder, 'how far the
earth's attraction, which has caused this hard blow, extends.' His
third, 'why not as far as the moon?' And he proceeds to assign the
motion of the moon to the same cause as that which produced the motion
of the apple. Taking this as a working hypothesis, he examines