Within thirty days, if someone forgets mashiv haru-ach, he’s supposed to repeat. If he’s unsure, he’s also supposed to repeat. Suppose he’s unsure but he does not repeat. Suppose, further, that in point of fact (kamei shemaya galya) he said it. Does he miss out?
When they said that someone who is unsure has to repeat, did they assimilate it to the case in which someone has no doubt that he failed to mention it, in which case they impose on him to repeat? Or did they not assimilate this case to that one, but instead said that this is what you do to resolve your doubt: take the stringent path and act as if you knew that you had not said it? On the first view, they assimilated the case of doubt to the case of certainty objectively, giving them the same status and, therefore, the same ruling. (Their thinking may have been that chances favor his not having said it. THEY thus resolved the question for him and rendered their ruling.) In the one case, like in the other, they obligated him to repeat. On the second view, by contrast, they did not pronounce on the objective status of the situation but, instead, addressed themselves to the subjective condition (characterized by a state of doubt) of the individual, instructing him how to go about in resolving this inner tension.
There is something to be said for the suggestion that on the resolution of this latter conundrum depends the resolution of the former.