but it
frequents trees, with whose bark its colour assimilates. The musk-sheep
is brown and conspicuous; but it is gregarious, and its safety depends
upon its ability to recognise its kind and keep with the herd. The raven
is always black; but it fears no enemy and feeds on carrion, and
therefore does not need concealment for either defence or attack. The
colour of the sable, then, though not white, serves for concealment; the
colour of the musk-sheep serves a purpose more important than
concealment; the raven needs no concealment. There are thus two sets of
instances:--in one set the animals are white (a) all the year, (b) in
winter; and white conceals them (a) all the year, (b) in winter; in the
other set, the animals are _not_ white, and to them either whiteness
would _not_ give concealment, or concealment would _not_ be
advantageous. And this second list refutes the rival hypotheses: for the
sable, the musk-sheep and the raven are as much exposed to the glare of
the snow, and to the cold, as the other animals are.

Sec. 3. THE CANON OF DIFFERENCE.

_If an instance in which a phenomenon occurs, and an instance in which
it does not occur, have every other circumstance in common save one,
that one (whether consequent or antecedent) occurring only in the
former; the circumstance in which alone the two instances differ is the
effect, or the cause, or an indispensable condition of the phenomenon._

This follows from Props. I (a) and (b), in chapter xv. Sec. 7. To prove
that A is a condition of _p_, let two instances, such as the Canon
requires, be represented thus:

A B C B C
_p q r_ _q r_

Then A is the cause or a condition of _p_. For, in the first instance, A
being introduced (without further change), _p_ arises (Prop. I. (a));
and, in the second instance, A having been removed (without other
change), _p_ disappears (Prop. I. (b)). Similarly we may prove, by the
same instances, that _p_ is the effect of A.

The order of the phenomena and the immediacy of their connection is a
matter for observation, aided by whatever instruments and methods of
inspection and measurement may be available.

As to the invariability of the connection, it may of course be tested by
collecting more instances or making more experiments; but it has been
maintained, that a single perfect experiment according to this method is
sufficient to prove causation, and therefore implies