tween the
_data_ of comparison and the subject of our inference. Like Deduction
and Induction, it assumes that things which are alike in some respects
are also alike in others; but it differs from them in not appealing to a
definite general law assigning the essential points of resemblance upon
which the argument relies. In Deductive proof, this is done by the major
premise of every syllogism: if the major says that 'All fat men are
humorists,' and we can establish the minor, 'X is a fat man,' we have
secured the essential resemblance that carries the conclusion. In
induction, the Law of Causation and its representatives, the Canons,
serve the same purpose, specifying the essential marks of a cause. But,
in Analogy, the resemblance relied on cannot be stated categorically.

If we argue that Mars is inhabited because it resembles the datum, our
Earth, (1) in being a planet, (2) neither too hot nor too cold for life,
(3) having an atmosphere, (4) land and water, etc., we are not
prepared to say that 'All planets having these characteristics are
inhabited.' It is, therefore, not a deduction; and since we do not know
the original causes of life on the Earth, we certainly cannot show by
induction that adequate causes exist in Mars. We rely, then, upon some
such vague notion of Uniformity as that 'Things alike in some points are
alike in others'; which, plainly, is either false or nugatory. But if
the linear markings upon the surface of Mars indicate a system of
canals, the inference that he has intelligent inhabitants is no longer
analogical, since canals can have no other cause.

The cogency of any proof depends upon the _character_ and _definiteness_
of the likeness which one phenomenon bears to another; but Analogy
trusts to the general _quantity_ of likeness between them, in ignorance
of what may be the really important likeness. If, having tried with a
stone, an apple, a bullet, etc., we find that they all break an
ordinary window, and thence infer that a cricket ball will do so, we do
not reason by analogy, but make instinctively a deductive extension of
an induction, merely omitting the explicit generalisation, 'All missiles
of a certain weight, size and solidity break windows.' But if, knowing
nothing of snakes except that the viper is venomous, a child runs away
from a grass-snake, he argues by analogy; and, though his conduct is
prudentially justifiable, his inference is wrong: for there is no law
that 'All snakes