Why the vision argument is a very good one!

[Angrboda]

The exact similarity of the cases [in an analogy] gives us a perfect assurance of a similar outcome; and we never want or look for stronger evidence than that. But the evidence is less strong when the cases are less than perfectly alike; any reduction in similarity, however tiny, brings a corresponding reduction in the strength of the evidence; and as we move down that scale we may eventually reach a very weak analogy, leading to a conclusion that is confessedly liable to error and uncertainty.

David Hume, Dialogues Concerning Natural Religion, p. II