t definitely assimilated to other causal
instances, but the effect is identified with the cause as the same
matter and energy redistributed; wherefore, whether the conservation of
matter and energy be universally true or not, it must still be an
universal postulate of scientific explanation.

The mere discovery of an empirical law of co-existence, as that 'white
tom-cats with blue eyes are deaf', is indeed something better than an
isolated fact: every general proposition relieves the mind of a load of
facts; and, for many people, to be able to say--'It is always so'--may
be enough; but for scientific explanation we require to know the reason
of it, that is, the cause. Still, if asked to explain an axiom, we can
only say, 'It is always so:' though it is some relief to point out
particular instances of its realisation, or to exhibit the similarity of
its form to that of other axioms--as of the _Dictum_ to the axiom of
equality.

Sec. 6. There are three modes of scientific Explanation; First, the
analysis of a phenomenon into the laws of its causes and the concurrence
of those causes.

The pumping of water implies (1) pressure of the air, (2) distribution
of pressure in a liquid, (3) that motion takes the direction of least
resistance. Similarly, that thunder follows forked lightning, and that
the report of a gun follows the flash, are resolvable into (1) the
discharge of electricity, or the explosion of gunpowder; (2) distance of
the observer from the event; (3) that light travels faster than sound.
The planetary orbits are analysable into the tendency of planets to fall
into the sun, and their tendency to travel in a straight line. When this
conception is helped out by swinging a ball round by a string, and then
letting it go, to show what would happen to the earth if gravitation
ceased, we see how the recognition of resemblance lies at the bottom of
explanation.

Secondly, the discovery of steps of causation between a cause and its
remote effects; the interpolation and concatenation of causes.

The maxim 'No cats no clover' is explained by assigning the intermediate
steps in the following series; that the fructification of red clover
depends on the visits of humble-bees, who distribute the pollen in
seeking honey; that if field-mice are numerous they destroy the
humble-bees' nests; and that (owls and weasels being exterminated by
gamekeepers) the destruction of field-mice depends upon the supply of
cats; which, th