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 phenomena, 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.