But let us examine the consequences of the really interesting proposition I have just quoted.  I presume that it is a law of nature that “a straight line is the shortest distance between two points.”  This law affirms the constant association of a certain fact of form with a certain fact of dimension.  Whether the notion of necessity which attaches to it has an a priori, or an a posteriori origin is a question not relevant to the present discussion.  But I would beg to be informed, if it is necessary, where is the “compelling force” out of which the necessity arises; and further, if it is not necessary, whether it loses the character of a law of nature?

I take it to be the law of nature, based on unexceptionable evidence, that the mass of matter remains unchanged, whatever chemical or other modifications it may undergo.  This law is one of the foundations of chemistry.  But it is by no means necessary.  It is quite 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