Sefer HaChinuch ספר החינוך

The commandment of the matter of the impurity of a carcass: That a carcass be impure and render impure, as it is stated (Leviticus 11:39), “And if a beast that is for you to eat, dies, etc.” Rambam, may his memory be blessed, wrote about this commandment (Sefer HaMitzvot, Positive Commandments 96), and this is his language: “And I will mention to you here now a nice preface, that you should remember regarding all that we mention of the types of impurities. And it is that that which we count each and every type of the impurities as a positive commandment, its substance is not that we would be obligated to become impure with this impurity, and likewise not that we are prevented from becoming impure from it and that it should be a negative commandment. Rather, since the Torah states that one who approaches this species becomes impure or that this thing [carcass] — in this situation — will make the one [or thing] who approaches it impure, that is a positive commandment — meaning to say that this law that we are commanded about is a commandment. And that [commandment] is that which we said, that the one who approaches such in this way becomes impure, and the one in that way does not become impure. And as to one becoming impure, himself, the option is in the hand of each man: As if he wants, he becomes impure; and if he wants, he does not become impure. And the language of Sifra, Shemini, Chapter 4:10 is ‘“And their carcass you shall not touch” — is it possible that if one touched a carcass he receives forty lashes? [Hence] we learn to say “and to these you shall become impure” (Leviticus 11:24). Is it possible that if one saw a carcass, he should go and become impure from it? [Hence] we learn to say “their carcass you shall not touch.” How is this? I would say [it is an] option.’ And the commandment is that which is said to us about these laws — that the one who approaches this becomes impure and will be impure; and he will be obligated that which the impure are obligated — to go out of the encampment of the Divine Presence and not to eat from the holy, and not to come close to it and other than this. And that is the commandment, meaning to say his being impure with this species, when he approaches it or be with it in this manner. And remember this matter with each and every type of impurity.” To here is his language. And with all of this, it does not sit well with the heart for us to think of this matter as a commandment. And nonetheless we shall not veer from the path of our rabbi in our tally to the right or the left, as we set out at the beginning.

The root is written above (Sefer HaChinukh 159).

From the laws of the commandment is that which they, may their memory be blessed, said (Mishnah Kelim 1:2) that a carcass is a primary source (av) from the primary sources of impurity; and that a kazayit of its flesh makes a man and vessels impure by touching, and a clay pot from its hollow; and makes a man impure from carrying to make [his] clothes impure. And it is one whether it is a permitted beast or animal or a forbidden one that died — the flesh of all of them transfers impurity with a kazayit. And the slaughter of a pure beast or animal purifies them and they do not transfer impurity afterwards. And even the slaughter of non-sacred [animals] in the courtyard or consecrated ones outside purifies them from being a carcass. And anything that dies on its own or a disqualification occurred in its slaughter, behold it is a carcass — and like they, may their memory be blessed, said in Chullin 32a, “This is the general principle: All that became disqualified in its slaughter is a carcass; and all that something else caused it to become disqualified, is ‘torn’ (tereifah).” But slaughter does not purify an impure beast or animal from being a carcass. And hence they are carcasses in any way that they die. And nonetheless, they do not transfer impurity — not them and not the pure ones — until they die completely, and not while they are still twitching. And the rest of its many details are elucidated scattered in the Order of Tahorot, but mostly in Tractate Kelim and Tractate Tahorot. (See Mishneh Torah, Laws of Defilement of Foods 1.)