It's a reasonable hypothesis that Kant came up with, but until he's tested it -- or at least thought of a way to test it -- he should have been more tentative about it.
Really? Why is the fact that you've thought of a way to test something a reason to be more confident of it?
I agree that if he had actually tested it that would have been reason for more confidence, but intention to experiment is not Bayesian evidence.
This is our monthly thread for collecting these little gems and pearls of wisdom, rationality-related quotes you've seen recently, or had stored in your quotesfile for ages, and which might be handy to link to in one of our discussions.