in nature we may generalise our
observation into a law, because that process is invariable. First, then,
can we observe the course of cause and effect? Our power to do so is
limited by the refinement of our senses aided by instruments, such as
lenses, thermometers, balances, etc. If the causal process is
essentially molecular change, as in the maintenance of combustion by
oxygen, we cannot directly observe it; if the process is partly cerebral
or mental, as in social movements which depend on feeling and opinion,
it can but remotely be inferred; even if the process is a collision of
moving masses (billiard-balls), we cannot really observe what happens,
the elastic yielding, and recoil and the internal changes that result;
though no doubt photography will throw some light upon this, as it has
done upon the galloping of horses and the impact of projectiles. Direct
observation is limited to the effect which any change in a phenomenon
(or its index) produces upon our senses; and what we believe to be the
causal process is a matter of inference and calculation. The meagre and
abstract outlines of Inductive Logic are apt to foster the notion, that
the evidence on which Science rests is simple; but it is amazingly
intricate and cumulative.

Secondly, so far as we can observe the process of nature, how shall we
judge whether a true causal instance, a relation of cause and effect,
is before us? By looking for the five marks of Causation. Thus, in the
experiment above described, showing that oxygen supports combustion, we
find--(1) that the taper which only glowed before being plunged into the
oxygen, bursts into flame when there--Sequence; (2) that this begins to
happen at once without perceptible interval--Immediacy; (3) that no
other agent or disturbing circumstance was present (the preparation of
the experiment having excluded any such thing)--Unconditionalness; (4)
the experiment may be repeated as often as we like with the same
result--Invariableness. Invariableness, indeed, I do not regard as
formally necessary to be shown, supposing the other marks to be clear;
for it can only be proved within our experience; and the very object of
Induction is to find grounds of belief beyond actual experience.
However, for material assurance, to guard against his own liability to
error, the inquirer will of course repeat his experiments.

The above four are the qualitative marks of Causation: the fifth and
quantitative mark is the Equ