The commandment of the impurity of a dead body: That we were commanded regarding the matter of impurity from a dead body to behave as the Torah commanded us about it, as it is stated (Numbers 19:14), “This is the law; a man who dies in a tent — anyone who comes into the tent and everything that is in the tent will become impure seven days.”
And I have written what I could from the roots of the matter of the impurity of the dead body in the Order of Emor el HaKohanim (Sefer HaChinukh 263); and so too, some of the laws — as is my custom — take it from there. And all the laws of the one rendered impure by a dead body are elucidated in Tractate Ohalot. (See Mishneh Torah, Laws of Defilement by a Corpse 1.) And in the Order of Vayehi Bayom Hashmini in the commandment of the impurity of the eight crawling animals (Sefer HaChinukh 159), you will also see it written that Ramban, may his memory be blessed, disagrees with Rambam, may his memory be blessed, not to count all the laws of the impurities in the tally of the commandments — and his reasoning is with him.
מִצְוַת טֻמְאָה שֶׁל מֵת – שֶׁנִּצְטַוִּינוּ לְהִתְנַהֵג בְּעִנְיַן טֻמְאַת מֵת כְּמוֹ שֶׁצִּוְּתָה אוֹתָנוּ הַתּוֹרָה עָלָיו, שֶׁנֶּאֱמַר (במדבר יט יד) זֹאת הַתּוֹרָה אָדָם כִּי יָמוּת בְּאֹהֶל כָּל הַבָּא אֶל הָאֹהֶל וְכָל אֲשֶׁר בָּאֹהֶל יִטְמָא שִׁבְעַת יָמִים.
The commandment of the impurity of a dead body: That we were commanded regarding the matter of impurity from a dead body to behave as the Torah commanded us about it, as it is stated (Numbers 19:14), “This is the law; a man who dies in a tent — anyone who comes into the tent and everything that is in the tent will become impure seven days.”
מִשָּׁרְשֵׁי עִנְיַן טֻמְאַת גּוּף הַמֵּת כָּתַבְתִּי בְּסֵדֶר אֱמֹר אֶל הַכֹּהֲנִים בְּמִצְוָה (רסג) מָה שֶׁיָּכֹלְתִּי, וּכְמוֹ כֵן קְצָת הַדִּינִים כְּמִנְהָגִי, וְקָחֶנּוּ מִשָּׁם. וּבְמַסֶּכֶת אֹהָלוֹת, יִתְבָּאֲרוּ כָּל דִּינֵי טֻמְאַת מֵת [ה' טומאת מת פ"א]. וּבְסֵדֶר וַיְהִי בַּיּוֹם הַשְּׁמִינִי בְּמִצְוַת טֻמְאַת שְׁמֹנָה שְׁרָצִים (מצוה קנט), תִּרְאֶה כָּתוּב גַּם כֵּן כִּי הָרַמְבַּ"ן זִכְרוֹנוֹ לִבְרָכָה יַחְלֹק עַל הָרַמְבַּ"ם זִכְרוֹנוֹ לִבְרָכָה שֶׁלֹּא לַחְשֹׁב כָּל דִּינֵי הַטֻּמְאוֹת בְּחֶשְׁבּוֹן הַמִּצְוֹת, וְנִמּוּקוֹ עִמּוֹ.
And I have written what I could from the roots of the matter of the impurity of the dead body in the Order of Emor el HaKohanim (Sefer HaChinukh 263); and so too, some of the laws — as is my custom — take it from there. And all the laws of the one rendered impure by a dead body are elucidated in Tractate Ohalot. (See Mishneh Torah, Laws of Defilement by a Corpse 1.) And in the Order of Vayehi Bayom Hashmini in the commandment of the impurity of the eight crawling animals (Sefer HaChinukh 159), you will also see it written that Ramban, may his memory be blessed, disagrees with Rambam, may his memory be blessed, not to count all the laws of the impurities in the tally of the commandments — and his reasoning is with him.