Full Idea
The Law of Causation, the recognition of which is the main pillar of inductive science, is but the familiar truth, that invariability of succession is found by observation between every fact in nature and some other fact which has preceded it.
Gist of Idea
Causation is just invariability of succession between every natural fact and a preceding fact
Source
John Stuart Mill (System of Logic [1843], 3.5.2), quoted by Bertrand Russell - On the Notion of Cause p.178
Book Reference
Russell,Bertrand: 'Mysticism and Logic' [Unwin 1989], p.178
A Reaction
Note that Mill rests causation on 'facts'. In the empiricist Mill endorsing the views of Hume. Russell attacks the bogus claim that science rests on causation. Personally I think Mill's view is incorrect.