12 On certain kinds of Groups or Series. [CHAP. I. 



stances and attributes. That which gives its unity to the 

 succession of groups is the fact of some of these substances or 

 attributes being common to the whole succession ; that which 

 gives their distinction to the groups in the succession is the 

 fact of some of them containing only a portion of these sub 

 stances and attributes, the other portion or portions being 

 occasionally absent. So understood, our phraseology may 

 be made to embrace every class of things of which Proba 

 bility can take account. 



8. It will be easily seen that the ordinary expression 

 (viz. the event, and the way in which it happens ) may be 

 included in the above. When the occasional attributes are 

 unimportant the permanent ones are sufficient to fix and 

 appropriate the name, the presence or absence of the others 

 being simply denoted by some modification of the name or 

 the addition of some predicate. We may therefore in all such 

 cases speak of the collection of attributes as &amp;lt; the event/ 

 the same event essentially, that is only saying that it (so as 

 to preserve its nominal identity) happens in different ways 

 in the different cases. When the occasional attributes how 

 ever are important, or compose the majority, this way of 

 speaking becomes less appropriate ; language is somewhat 

 strained by our implying that two extremely different assem 

 blages are in reality the same event, with a difference only 

 in its mode of happening. The phrase is however a very 

 convenient one, and with this caution against its being mis 

 understood, it will frequently be made use of here. 



9. A series of the above-mentioned kind is, I ap 

 prehend, the ultimate basis upon which all the rules of 

 Probability must be based. It is essential to a clear com 

 prehension of the subject to have carried our analysis up 

 to this point, but any attempt at further analysis into the 

 intimate nature of the events composing the series, is not 



