In logic if we have X implies Y and you tell me Y is true then I cannot say X is true. That's because true -> true and false -> true are both true statements. Knowing that Y is true doesn't tell me anything about X.

But now Bayes wants to say that if we know Y is true then X is more likely? Note that the case where X is false is know as the vacuous truth which is often confusing and trips up beginners in logic. Perhaps Bayes did not know logic so he got confused here?