na, yet it can never be a methodical or logical proof of
causation, since it does not indicate the unconditionalness of the
sequence. For simple enumeration of the sequence A-_p_ leaves open the
possibility that, besides A, there is always some other antecedent of
_p_, say X; and then X may be the cause of _p_. To disprove it, we must
find, or make, a negative instance of X--where _p_ occurs, but X is
absent.

So far as we recognise the possibility of a plurality of causes, this
method of Agreement cannot be quite satisfactory. For then, in such
instances as the above, although D is absent in the first, and B in the
second, it does not follow that they are not the causes of _p_; for they
may be alternative causes: B may have produced _p_ in the first
instance, and D in the second; A being in both cases an accidental
circumstance in relation to _p_. To remedy this shortcoming by the
method of Agreement itself, the only course is to find more instances of
_p_. We may never find a negative instance of A; and, if not, the
probability that A is the cause of _p_ increases with the number of
instances. But if there be no antecedent that we cannot sometimes
exclude, yet the collection of instances will probably give at last all
the causes of _p_; and by finding the proportion of instances in which
A, B, or X precedes _p_, we may estimate the probability of any one of
them being the cause of _p_ in any given case of its occurrence.

But this is not enough. Since there cannot really be vicarious causes,
we must define the effect (_p_) more strictly, and examine the cases to
find whether there may not be varieties of _p_, with each of which one
of the apparent causes is correlated: A with _p_^{1} B with _p_^{11}, X
with _p_^{111}. Or, again, it may be that none of the recognised
antecedents is effective: as we here depend solely on observation, the
true conditions may be so recondite and disguised by other phenomena as
to have escaped our scrutiny. This may happen even when we suppose that
the chief condition has been isolated: the drinking of foul water was
long believed to cause dysentery, because it was a frequent antecedent;
whilst observation had overlooked the bacillus, which was the
indispensable condition.

Again, though we have assumed that, in the instances supposed above,
immediate sequence is observable, yet in many cases it may not be so, if
we rely only on the canon of Agreement; if instances cannot be obtained
by