tands this without our
mentioning it. Nor do we consider it necessary to mention the acoustic
condition, that the vibration of the bells is communicated to our ears
through the air, or the physiological condition, that the vibrations
in the drums of our ears are conveyed by a certain mechanism of bone
and tissue to the nerves. Our hearer may not care to know this,
though quite prepared to admit that these conditions are indispensable
antecedents. Similarly, a physiographer, in stating the cause of the
periodical inundation of the Nile, would consider it enough to mention
the melting of snow on the mountains in the interior of Africa,
without saying anything of such conditions as the laws of gravity
or the laws of liquefaction by heat, though he knows that these
conditions are also indispensable. Death is explained by the doctor
when referred to a gunshot wound, or a poison, or a virulent disease.
The Pathologist may inquire further, and the Moral Philosopher further
still. But all inquiries into indispensable conditions are inquiries
into cause. And all alike have to be on their guard against mistaking
simple sequence for consequence.

To speak of the sum total of conditions, as the Cause in a
distinctively scientific sense, is misleading in another direction. It
rather encourages the idea that science investigates conditions in
the lump, merely observing the visible relations between sets of
antecedents and their consequents. Now this is the very thing
that science must avoid in order to make progress. It analyses the
antecedent situation, tries to separate the various coefficients, and
finds out what they are capable of singly. It must recognise that
some of the antecedents of which it is in search are not open to
observation. It is these, indeed, for the most part that constitute
the special subject-matter of the sciences in Molar as well as in
Molecular Physics. For practical every-day purposes, it is chiefly
the visible succession of phenomena that concerns us, and we are
interested in the latent conditions only in as far as they provide
safer ground for inference regarding such visible succession. But to
reach the latent conditions is the main work of science.

It is, however, only through observation of what is open to the senses
that science can reach the underlying conditions, and, therefore,
to understand its methods we must consider generally what is open to
observation in causal succession. What can be obs