erved when phenomena
follow one another as cause and effect, that is, when the one happens
in consequence of the happening of the other? In Hume's theory,
which Mill formally adopted with a modification,[3] there is nothing
observable but the constancy or invariability of the connexion. When
we say that Fire burns, there is nothing to be observed except that a
certain sensation invariably follows upon close proximity to fire. But
this holds good only if our observation is arbitrarily limited to the
facts enounced in the expression. If this theory were sound, science
would be confined to the observation of empirical laws. But that there
is something wrong with it becomes apparent when we reflect that it
has been ascertained beyond doubt that in many observed changes, and
presumably in all, there is a transference of energy from one form to
another. The paralogism really lies in the assumption from which
Hume deduced his theory, namely, that every idea is a copy of some
impression. As a matter of fact, we have ideas that are not copies
of any one impression, but a binding together, colligation, or
intellection of several impressions. Psychological analysis shows us
that even when we say that things exist with certain qualities, we are
expressing not single impressions or mental phenomena, but supposed
causes and conditions of such, _noumena_ in short, which connect our
recollections of many separate impressions and expectations of more.

The Experimental Methods proceed on the assumption that there is other
outward and visible evidence of causal connexion than invariability of
sequence. In the leading Method it is assumed that when events may be
observed to follow one another in a certain way, they are in causal
sequence. If we can make sure that an antecedent change is the only
change that has occurred in an antecedent situation, we have proof
positive that any immediately subsequent change in the situation is a
consequent, that the successive changes are in causal sequence. Thus
when Pascal's barometer was carried to the top of Puy le Dome, and
the mercury in it fell, the experimenters argued that the fall of the
mercury was causally connected with the change of elevation, all the
other circumstances remaining the same. This is the foundation of the
so-called Method of Difference. To determine that the latent condition
was a difference in the weight of the atmosphere, needed other
observations, calculations and inferenc