Introduction Leviticus 27:10 states, “If one substitutes one animal for another, the thing vowed and its substitute shall both be holy.” This teaches that if one has an animal already dedicated to the Temple to be offered as a sacrifice and he says concerning another animal, “Let this one be its substitute”, the first animal remains sacred and its substitute becomes sacred as well (there is a whole mishnaic tractate about substitutes). Our mishnah deals with someone who tries to offer a substitute for a pesah. In order to understand the mishnah we also need to know that animals dedicated to be pesahim which are not offered as such are offered later as offerings of wellbeing.
Rabbi Joshua said: I have heard that the substitute of a pesah is sacrificed, and that the substitute of a pesah is not sacrificed, and I cannot explain. Rabbi Joshua says that he has heard a puzzling tradition which he does not understand. He has heard that sometimes the substitute of a pesah is sacrificed as a thanksgiving offering and sometimes it is not, but he doesn’t know how to explain this tradition. As an aside, we should note that this statement provides us with an interesting glimpse into how the rabbinic tradition was transmitted and developed. Rabbi Joshua has heard a statement, has memorized it, but didn’t receive along with the statement any interpretation and hence doesn’t know what it means. The statement was brief and probably easy to memorize but was difficult to understand.
Rabbi Akiva said: I will explain: The pesah which was found before the slaughtering of the pesah must be left to graze until it becomes unfit, and then it may be sold, and one brings a peace-offering with the money; and the same applies to its substitute. [If found] after the slaughtering of the pesah, it is offered as a wellbeing offering, and the same applies to it substitute. Rabbi Akiva uses his reason to figure out this conundrum. If it happens that a lamb/kid set aside to be a pesah is lost and another is set aside to be offered in its place (this is not the substitute, but a replacement), and the original one is found before the replacement is sacrificed then the replacement is not offered. Rather it is left to graze until it can no longer be offered as a pesah (until it becomes flawed), then it is sold and then the proceeds from the sale are used to buy another animal to serve as a wellbeing offering. It itself cannot be sacrificed because it existed when the pesah was offered but it itself was rejected as a pesah and not sacrificed. If a person makes a substitute for this second pesah then the substitute is also not offered. The substitute has the same rules as that for which it was substituted. This is the substitute which is not sacrificed. If it happens that the lost pesah is not found until after the new one has been sacrificed, the original pesah is offered as a sacrifice of wellbeing. So too, if one makes a substitute for this original pesah, it too is offered as a sacrifice of wellbeing. This is the substitute which is sacrificed.