is good when used by experts, whose familiarity with the subject makes
it good.

Sec. 4. THE CANON OF CONCOMITANT VARIATIONS.

_Whatever phenomenon varies in any manner whenever another phenomenon
(consequent or antecedent) varies in some particular manner [no other
change having concurred] is either the cause or effect of that
phenomenon [or is connected with it through some fact of causation]._

This is not an entirely fresh method, but may be regarded as a special
case either of Agreement or of Difference, to prove the cause or effect,
not of a phenomenon as a whole, but of some increment of it (positive or
negative). There are certain forces, such as gravitation, heat,
friction, that can never be eliminated altogether, and therefore can
only be studied in their degrees. To such phenomena the method of
Difference cannot be applied, because there are no negative instances.
But we may obtain negative instances of a given quantity of such a
phenomenon (say, heat), and may apply the method of Difference to that
quantity. Thus, if the heat of a body increases 10 degrees, from 60 to
70, the former temperature of 60 was a negative instance in respect of
those 10 degrees; and if only one other circumstance (say, friction)
has altered at the same time, that circumstance (if an antecedent) is
the cause. Accordingly, if in the above Canon we insert, after
'particular manner,' "[no other change having concurred,]" it is a
statement of the method of Difference as applicable to the increment of
a phenomenon, instead of to the phenomenon as a whole; and we may then
omit the last clause--"[or is connected, etc.]." For these words are
inserted to provide for the case of co-effects of a common cause (such
as the flash and report of a gun); but if no other change (such as the
discharge of a gun) has concurred with the variations of two phenomena,
there cannot have been a common cause, and they are therefore cause and
effect.

If, on the other hand, we omit the clause "[no other change having
concurred,]" the Canon is a statement of the method of Agreement as
applicable to the increment of a phenomenon instead of to the phenomenon
as a whole; and it is then subject to the imperfections of that method:
that is to say, it leaves open the possibilities, that an inquirer may
overlook a plurality of causes; or may mistake a connection of two
phenomena, which (like the flash and report of a gun) are co-effects of
a common cause, for a d