urrence of A, an antecedent, and _p_, a consequent, with concomitant
facts or events--and let us represent them thus:

Antecedents: A B C A D E
Consequents: _p q r_ _p s t_;

and suppose further that, in this case, the immediate succession of
events can be observed. Then A is probably the cause, or an
indispensable condition, of _p_. For, as far as our instances go, A is
the invariable antecedent of _p_; and _p_ is the invariable consequent
of A. But the two instances of A or _p_ agree in no other circumstance.
Therefore A is (or completes) the unconditional antecedent of _p_. For B
and C are not indispensable conditions of _p_, being absent in the
second instance (Rule II. (b)); nor are D and E, being absent in the
first instance. Moreover, _q_ and _r_ are not effects of A, being absent
in the second instance (Rule II. (d)); nor are _s_ and _t_, being absent
in the first instance.

It should be observed that the cogency of the proof depends entirely
upon its tending to show the unconditionality of the sequence A-_p_, or
the indispensability of A as a condition of _p_. That _p_ follows A,
even immediately, is nothing by itself: if a man sits down to study and,
on the instant, a hand-organ begins under his window, he must not infer
malice in the musician: thousands of things follow one another every
moment without traceable connection; and this we call 'accidental.' Even
invariable sequence is not enough to prove direct causation; for, in
our experience does not night invariable follow day? The proof requires
that the instances be such as to show not merely what events _are_ in
invariable sequence, but also what _are not_. From among the occasional
antecedents of _p_ (or consequents of A) we have to eliminate the
accidental ones. And this is done by finding or making 'negative
instances' in respect of each of them. Thus the instance

A D E
_p s t_

is a negative instance of B and C considered as supposable causes of _p_
(and of _q_ and _r_ as supposable effects of A); for it shows that they
are absent when _p_ (or A) is present.

To insist upon the cogency of 'negative instances' was Bacon's great
contribution to Inductive Logic. If we neglect them, and merely collect
examples of the sequence A-_p_, this is 'simple enumeration'; and
although simple enumeration, when the instances of agreement are
numerous enough, may give rise to a strong belief in the connection of
phenome