rential extension of uniformities such
as that death is common to all organised beings. One of the Methods,
as we shall see, that named by Mill the Method of Agreement, does
incidentally and collaterally establish empirical laws in the course
of its observations, and this probably accounts for the prominence
given to it in Mill's system. But this is not its end and aim, and
the leading Method, that named by him the Method of Difference,
establishes as fact only a particular case of causal coincidence.
It is with the proof of theories of causation that the Experimental
Methods are concerned: they are methods of observing with a view to
such proof.[1]

The next point to be made clear is that the facts of causation with
which the Methods are concerned are observable facts, relations among
phenomena, but that the causal relations or conditions of which they
are the proof are not phenomena, in the meaning of being manifest
to the senses, but rather noumena, inasmuch as they are reached by
reasoning from what is manifest.

Take, for example, what is known as the _quaquaversus_ principle in
Hydrostatics, that pressure upon a liquid is propagated equally in
all directions. We cannot observe this extension of pressure among the
liquid particles directly. It cannot be traced among the particles
by any of our senses. But we can assume that it is so, consider what
ought to be visible if it is so, and then observe whether the visible
facts are in accordance with the hypothesis. A box can be made, filled
with water, and so fitted with pistons on top and bottom and on each
of its four sides that they will indicate the amount of pressure on
them from within. Let pressure then be applied through a hole in
the top, and the pistons show that it has been communicated to them
equally. The application of the pressure and the yielding of the
pistons are observable facts, facts in causal sequence: what happens
among the particles of the liquid is not observed but reasonably
conjectured, is not _phenomenal_ but _noumenal_.

This distinction, necessary to an understanding of the scope of the
Methods, was somewhat obscured by Mill in his preliminary
discussion of the meaning of "cause". Very rightly, though somewhat
inconsistently with his first theory of Induction, he insists that
"the notion of Cause being the root of the whole theory of Induction,
it is indispensable that this idea should at the very outset of our
inquiry be, with the utmo