would be
intelligible: an ancient coin or weapon would have no meaning, were we
not acquainted with the origins and uses of other coins and weapons.
Generally, the further we go back in history, the more the evidence
needs interpretation and reconstruction, and the more prominent becomes
the appeal to the Comparative Method. Our aim is to construct a history
of the world, and of the planet as part of the world, and of mankind as
part of the life of the planet, in such a way that every event shall be
consistent with, and even required by, the rest according to the
principle of Causation.

CHAPTER XVIII

HYPOTHESES

Sec. 1. An Hypothesis, sometimes employed instead of a known law, as a
premise in the deductive investigation of nature, is defined by Mill as
"any supposition which we make (either without actual evidence, or on
evidence avowedly insufficient) in order to endeavour to deduce from it
conclusions in accordance with facts which are known to be real; under
the idea that if the conclusions to which the hypothesis leads are known
truths, the hypothesis itself either must be, or at least is likely to
be, true." The deduction of known truths from an hypothesis is its
Verification; and when this has been accomplished in a good many cases,
and there are no manifest failures, the hypothesis is often called a
Theory; though this term is also used for the whole system of laws of a
certain class of phenomena, as when Astronomy is called the 'theory of
the heavens.' Between hypothesis and theory in the former sense no
distinct line can be drawn; for the complete proof of any speculation
may take a long time, and meanwhile the gradually accumulating evidence
produces in different minds very different degrees of satisfaction; so
that the sanguine begin to talk of 'the theory,' whilst the circumspect
continue to call it 'the hypothesis.'

An Hypothesis may be made concerning (1) an Agent, such as the ether; or
(2) a Collocation, such as the plan of our solar system--whether
geocentric or heliocentric; or (3) a Law of an agent's operation, as
that light is transmitted by a wave motion of such lengths or of such
rates of vibration.

The received explanation of light involves both an agent, the ether, as
an all-pervading elastic fluid, and also the law of its operation, as
transmitting light in waves of definite form and length, with definite
velocity. The agreement between the calculated results of this complex
hy