that a system of philosophy breaks down and fails, then we
may nearly always be sure its defect will reveal itself as an
unreconciled dualism. Such a philosophy will begin with
a monistic principle, and will attempt to derive or deduce
the entire universe from it, but somewhere or other it comes
across something in the world which it cannot bring under
that principle. Then it is left with two equally ultimate
existences, neither of which can be derived from the other.
Thus it breaks out into dualism.
Now the search for a monistic explanation of things is a
universal tendency of human thought. Wherever we look in
the world of thought, we find that this monistic tendency
appears. I have already said that it appears throughout
the history of philosophy. It reveals itself, {64} too, very
clearly in the history of religion. Religion begins in poly-
theism, the belief in many gods. From that it passes on
to monotheism, the belief in one God, who is the sole au-
thor and creator of the universe. In Hindu thought we find
the same thing. Hindu thought is based upon the princi-
ple that “All is one.” Everything in the world is derived
from one ultimate being, Brahman. But not only is this
monistic tendency traceable in religion and philosophy; it
is also traceable in science. The progress of scientific ex-
planation is essentially a progress towards monism. In the
first place, the explanation of isolated facts consists always
in assigning causes for them. Suppose there is a strange
noise in your room at night. You say it is explained when
you find that it is due to the falling of a book or the scut-
tling of a rat across the floor. The noise is thus explained
by assigning a cause for it. But this simply means that you
have robbed it of its isolated and exceptional position, and
reduced it to the position of an example of a general law.
When the water freezes in your jug, you say that the cause
of this is the cold. It is an example of the law that whenever
the cold reaches a certain degree, then, other things being
equal, water solidifies. But to assign causes in this way is
not really to explain anything. It does not give any reason
for an event happening. You cannot see any reason why
water should solidify in the cold. It merely tells us that the
event is not exceptional, but is an example of what always
happens. It reduces the isolated event to a case of a general
law, which “explains,” not merely this one event, but pos-
sibly millions of events. It is not merely that cold solidifies
the water in your jug. {65} It equally solidifies the water
in everybody’s jug. The same law “explains” all these, and
likewise “explains” icebergs and the polar caps on the earth
and the planet Mars. In fact scientific explanation means
the reduction of millions of facts to one principle. But sci-
ence does not stop here. It seeks further to explain the laws
themselves, and its method is to reduce the many laws to
one higher and more general law. A familiar example of
this is the explanation of Kepler’s laws of the planetary
motions. Kepler laid down three such laws. The first was
that planets move in elliptical orbits with the sun in one
focus. The second was that planets describe equal areas in
equal times. The third was a rather more complicated law.
Kepler knew these laws from observation, but he could not
explain them. They were explained by Newton’s discov-
ery of the law of gravitation. Newton proved that Kepler’s
three laws could be mathematically deduced from the law
of gravitation. In that way Kepler’s laws were explained,