e possible to
imagine that the mass of matter should vary according to
circumstances, as we know its weight does. Moreover, the determination
of the "force" which makes mass constant (if there is any
intelligibility in that form of words) would not, so far as I can see,
confer any more validity on the law than it has now.

There is a law of nature, so well vouched by experience, that all
mankind, from pure logicians in search of examples to parish sextons
in search of fees, confide in it. This is the law that "all men are
mortal." It is simply a statement of the observed order of facts that
all men sooner or later die. I am not acquainted with any law of
nature which is more "constant and uniform" than this. But will any
one tell me that death is "necessary"? Certainly there is no _a
priori_ necessity in the case, for various men have been imagined to
be immortal. And I should be glad to be informed of any "necessity"
that can be deduced from biological considerations. It is quite
conceivable, as has recently been pointed out, that some of the lowest
forms of life may be immortal, after a fashion. However this may be, I
would further ask, supposing "all men are mortal" to be a real law of
nature, where and what is that to which, with any propriety, the title
of "compelling force" of the law can be given?

On page 69, the Duke of Argyll asserts that the law of gravitation "is
a law in the sense, not merely of a rule, but of a cause." But this
revival of the teaching of the "Vestiges" has already been examined
and disposed of; and when the Duke of Argyll states that the "observed
order" which Kepler had discovered was simply a necessary consequence
of the force of "gravitation," I need not recapitulate the evidence
which proves such a statement to be wholly fallacious. But it may be
useful to say, once more, that, at this present moment, nobody knows
anything about the existence of a "force" of gravitation apart from
the fact; that Newton declared the ordinary notion of such force to be
inconceivable; that various attempts have been made to account for the
order of facts we call gravitation, without recourse to the notion of
attractive force; that, if such a force exists, it is utterly
incompetent to account for Kepler's laws, without taking into the
reckoning a great number of other considerations; and, finally, that
all we know about the "force" of gravitation, or any other so-called
"force," is that it is a name for