Full Idea
Probability has two aspects: the degree of belief warranted by evidence, and the tendency displayed by some chance device to produce stable relative frequencies. These are the epistemological and statistical aspects of the subject.
Gist of Idea
Probability is statistical (behaviour of chance devices) or epistemological (belief based on evidence)
Source
Ian Hacking (The Emergence of Probability [1975], Ch.1)
Book Reference
Hacking,Ian: 'The Emergence of Probability' [CUP 1975], p.1
A Reaction
The most basic distinction in the subject. Later (p.124) he suggests that the statistical form (known as 'aleatory' probability) is de re, and the other is de dicto.