Full Idea
But wherever you depart, in the least, from the similarity of the cases, you diminish proportionably the evidence; and may at last bring it to a very weak analogy.
Gist of Idea
An analogy begins to break down as soon as the two cases differ
Source
David Hume (Dialogues Concerning Natural Religion [1751], Part 2)
Book Reference
Hume,David: 'Dialogues Concerning Natural Religion' [Penguin], p.54