hat air
is an indispensable condition of life. Hence, however much the moon
may resemble the earth, we are debarred from concluding that there are
living creatures on the moon such as we know to exist on the earth. We
know also that life such as it is on the earth is possible only within
certain limits of temperature, and that Mercury is too hot for life,
and Saturn too cold, no matter how great the resemblance to the earth
in other respects.

4. If the property inferred is known or presumed to be a concomitant
of one or more of the points of resemblance, any argument from analogy
is superfluous. This is, in effect, to say that we have no occasion to
argue from general resemblance when we have reason to believe that a
property follows from something that an object is known to possess.
If we knew that any one of the planets possessed all the conditions,
positive and negative, of life, we should not require to reckon up
all the respects in which it resembles the earth in order to create
a presumption that it is inhabited. We should be able to draw the
conclusion on other grounds than those of analogy. Newton's famous
inference that the diamond is combustible is sometimes quoted as an
argument from analogy. But, technically speaking, it was rather,
as Professor Bain has pointed out, of the nature of an extended
generalisation. Comparing bodies in respect of their densities and
refracting powers, he observed that combustible bodies refract more
than others of the same density; and observing the exceptionally high
refracting power of the diamond, he inferred from this that it was
combustible, an inference afterwards confirmed by experiment. "The
concurrence of high refracting power with inflammability was an
empirical law; and Newton, perceiving the law, extended it to the
adjacent case of the diamond. The remark is made by Brewster that had
Newton known the refractive powers of the minerals _greenockite_ and
_octohedrite_, he would have extended the inference to them, and would
have been mistaken."[2]

From these conditions it will be seen that we cannot conclude with any
high degree of probability from analogy alone. This is not to deny, as
Mr. Jevons seems to suppose, that analogies, in the sense of general
resemblances, are often useful in directing investigation. When
we find two things very much alike, and ascertain that one of them
possesses a certain property, the presumption that the other has
the same is strong