iven by Prof. Fowler there is not really a compliance with the
theoretical requirements of Mill's Method: there is only an increased
presumption from the double agreement. "The Joint Method of Agreement
and Difference (or the Indirect Method of Difference, or, as I
should prefer to call it, the Double Method of Agreement) is being
continually employed by us in the ordinary affairs of life. If when I
take a particular kind of food, I find that I invariably suffer from
some particular form of illness, whereas, when I leave it off, I cease
to suffer, I entertain a double assurance that the food is the cause
of my illness. I have observed that a certain plant is invariably
plentiful on a particular soil; if, with a wide experience, I fail to
find it growing on any other soil, I feel confirmed in my belief that
there is in this particular soil some chemical constituent, or
some peculiar combination of chemical constituents, which is highly
favourable, if not essential, to the growth of the plant."

[Footnote 1: Elimination, or setting aside as being of no
concern, must not be confounded with the exclusion of agents
practised in applying the Method of Difference. We use the
word in its ordinary sense of putting outside the sphere of
an argument. By a curious slip, Professor Bain follows Mill in
applying the word sometimes to the process of singling out or
disentangling a causal circumstance. This is an inadvertent
departure from the ordinary usage, according to which
elimination means discarding from consideration as being
non-essential.]

[Footnote 2: Hirsch's _Geographical and Historical Pathology_,
Creighton's translation, vol. ii. pp. 121-202.]

[Footnote 3: The bare titles Difference and Agreement, though
they have the advantage of simplicity, are apt to puzzle
beginners inasmuch as in the Method of Difference the
agreement among the instances is at a maximum, and the
difference at a minimum, and _vice versa_ in the Method of
Agreement. In both Methods it is really the isolation of the
connexion between antecedent and sequent that constitutes the
proof.]

[Footnote 4: That rainbows in the sky are produced by the
passage of light through minute drops in the clouds was an
inference from this observed uniformity.]

CHAPTER VI.

METHODS OF OBSERVATION.--MINOR METHODS.

I.--CONCOMITANT VARIATIONS.

_Whatever phe