כל עוף טהור תאכלו. וְלֹא אֶת הַטָּמֵא, בָּא לִתֵּן עֲשֵׂה עַל לֹא תַעֲשֶׂה; וְכֵן בַּבְּהֵמָה "אֹתָהּ תֹּאכֵלוּ", וְלֹא בְהֵמָה טְמֵאָה, וְלָאו הַבָּא מִכְּלַל עֲשֵׂה עֲשֵׂה, לַעֲבֹר עֲלֵיהֶם בַּעֲשֵׂה וְלֹא תַעֲשֶׂה: כל עוף תאכלו ALL CLEAN FOWLS, YOU MAY EAT — but not (as is implied by these words) the unclean. Scripture intends, by this statement, to attach to the negative command which forbids unclean fowls (v. 12), a positive one which implicitly contains a prohibition. And similarly, when in the case of clean cattle it states, (v. 6) “that you may eat”, it implies: not, however, the unclean ones. Now a prohibition which is not plainly expressed but can only be drawn by inference from a positive command, is itself regarded only as a positive command, so that one who eats such food transgresses thereby not two negative commands, but a positive and a negative command (Sifrei Devarim 101:10; and cf. Rashi on Leviticus 11:3).