cause,
Hirsch came to the conclusion that the true cause must be a morbid
poison, and that endemic goitre has to be reckoned among the
infectious diseases.[2]

On this negative principle, that if a circumstance comes and goes
without bringing the phenomenon in its train, the phenomenon
is causally independent of it, common-sense is always at work
disconnecting events that are occasionally coincident in time. A
bird sings at our window, for example, and the clock ticks on the
mantelpiece. But the clock does not begin to tick when the bird begins
to sing, nor cease to tick when the bird flies away. Accordingly, if
the clock should stop at any time, and we wished to inquire into the
cause, and anybody were to suggest that the stoppage of the clock was
caused by the stoppage of a bird's song outside, we should dismiss
the suggestion at once. We should eliminate this circumstance from our
inquiry, on the ground that from other observations we knew it to be
a casual or fortuitous concomitant. Hotspur's retort to Glendover
(p. 297) was based on this principle. When poetic sentiment or
superstition rejects a verdict of common-sense or science, it is
because it imagines a causal connexion to exist that is not open to
observation, as in the case of the grandfather's clock which stopped
short never to go again when the old man died.

II.--THE PRINCIPLE OF SINGLE AGREEMENT.

The procedure in Mill's "Method of Agreement" consists in thus
eliminating fortuitous antecedents or concomitants till only one
remains. We see the nature of the proof relied upon when we ask, How
far must elimination be carried in order to attain proof of causal
connexion? The answer is that we must go on till we have eliminated
all but one. We must multiply instances of the phenomenon, till we
have settled of each of the antecedents except one that it is not the
cause. We must have taken account of all the antecedents, and we
must have found in our observations that all but one have been only
occasionally present.

_When all the antecedents of an effect except one can be
absent without the disappearance of the effect, that one is
causally connected with the effect, due precautions being
taken that no other circumstances have been present besides
those taken account of._

Mill's Canon of the Method of Agreement is substantially identical
with this:--

When two or more instances of the phenomenon under
investigation have