brown throughout the winter; 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 invariability (since causation is uniform), though no other instances should ever be obtainable; because it establishes once for all the unconditionality of the connection

     A B C
    p q r.

Now, formally this is true; but in any actual investigation how shall we decide what is a satisfactory or perfect experiment?  Such an experiment requires that in the negative instance