Full Idea
The whole chain or succession [of causes and effects], taken together, is not caused by anything, and yet it is evident that it requires a cause or reason, as much as any particular object which begins to exist in time.
Gist of Idea
A chain of events requires a cause for the whole as well as the parts, yet the chain is just a sum of parts
Source
David Hume (Dialogues Concerning Natural Religion [1751], Part 9)
Book Reference
'The Existence of God', ed/tr. Hick,John [Macmillan 1964], p.95
A Reaction
This is such a major and significant idea. With blinkers on we think our questions are answered. Then someone (a philosopher, inevitably) makes you pull back and ask a much wider and more difficult question.