Sotah 28 - Laws of purity
Laws of purity are learned from the laws of suspected wife. Here's how. The reason her purity is in doubt is because she went into a private hiding place with a certain man. But if she was talking with him in public, her purity would not be in doubt.
In the same way, if there is a doubt about some other purity, such as, for example, a man who might have touched a dead rat, and the question now is whether he is considered ritually impure - if this happened in the street, he is declared pure, and if in a cave, then he is impure. This is a special law that goes beyond the regular rule of "let's treat it as a doubt." Here there is no doubt, and he is definitely pure in the street but impure in the cave.
Another law can be learned by noticing that a suspected wife is a human being who can be asked about her status. So with other situations, the distinction between happenings in the street and in a cave applies only to a human who can be asked. For example, he is not sure whether he touched a rat or no. Then the rule about the street or cave applies. But if he is sure that he touched the rat, and the doubt is about the rat itself, such as whether it was alive at this time or already dead - then he is always declared pure, because a rat cannot be asked.
Art: Two Rats by Vincent Van Gogh