What must be done when in doubt about contracting ritual impurity (najasah) or in doubt about the ritual purity of an object?

The author wants to clarify two issues:

First: Whoever doubts that impurity has afflicted him should sprinkle water on the spot about which he is in doubt. The doubt here is regarding the affliction, not the impurity of what caused it. He is certain that it is impure, but he doubts its affliction.

Second: If something afflicts him, and he doubts its impurity, then he does not need to sprinkle water; because the affliction here is certain, but the doubt is regarding the impurity of what caused it. And it is even more so that sprinkling water is not obligatory if he doubts both the affliction and the impurity of what caused it.