Halakhah for Eruvin 191:8

In this prevention of wine libations, both Rambam (in Sefer HaMitzvot LaRambam, Mitzvot Lo Taase 194), may his memory be blessed, and Ramban (in his gloss to the Sefer HaMitzvot), may his memory be blessed, conceded that there is a negative commandment in this and that it is counted in the tally of the negative commandments. However they did disagree about it in the [following] matter: that Rambam extracts the prohibition of wine libations from the verse that is written in Parshat Haazinu (Deuteronomy 32:38), "drank the wine of their libations"; and the prohibition of other gifts to idolatry from, "Nothing is to cling to your hand from the anathema"(Deuteronomy 13:18) and from "You shall not bring an abomination" (Deuteronomy 7:26); [whereas] Ramban, may his memory be blessed, wrote that we learn the prohibition of all of the gifts of idolatry from this verse of "Guard yourself," and wine libations are included. And I have written this verse, like his opinion - not like my custom in all of the book, as I have written all [of the other verses] according to the opinion of Rambam, may his memory be blessed. But in this [case] I saw that this verse is very fit to expound the matter from it; and also that there is a warning in it. And [it is] as they, may their memory be blessed, said (Eruvin 96a), [that] every place where it states, "guard yourself," or "lest" or "do not," it is nothing but a negative commandment. However in the verse, "drank the wine of their libations," there is no warning. And I also saw great ones from the enumerators of the commandment that wrote like this.

To not nullify the guarding of the Temple: To not nullify the guarding of the Temple, [but rather] to always walk around it every night, as it is stated (Numbers 18:5), "And you shall guard the guarding of the Holy." And it is well-known that the expression of guarding takes the place of a negative commandment. And [it is] as they, may their memory be blessed, said (Eruvin 96a), "Every place in which it is stated, 'guard yourself,' 'lest,' and 'not,' is nothing but a negative commandment." And they said in Mekhilta, "'And they shall guard the guarding of the Tent of Meeting' (Numbers 18:4), - I only [know of] a positive commandment. From where [do I know] a negative commandment? [Hence], we learn to say, 'And you shall guard the guarding of the Holy.'" To here [are the words of Mekhilta]. And maybe they expounded a positive commandment and a negative commandment when they found two verses teaching one thing. And from the one that comes by way of a command in second person, they learned a negative commandment; and they expounded the command in third person - which is lighter than it - about a positive commandment. And anyone who weighs the things in just scales will concede about this - that the third person is lighter than a command in second person.

To not eat forbidden mixtures of the vineyard: That we are forbidden from eating forbidden mixtures of the vineyard only. And we have already explained in the previous commandment what forbidden mixtures of the vineyard are. And about this is it stated (Deuteronomy 22:9), "lest it be set aside (tikdash), the crop, the seed that you planted and the produce of the vineyard." - and they, may their memory be blessed, explained (Kiddushin 56b), "Lest it be incinerated by fire (tukad esh)," which is to say that it is not fitting that there be a purpose [from it], as it should all be forbidden in benefit. And the proof that there is a negative commandment in it, is that it is written in its prevention, "lest (pen)." And they, may their memory be blessed, said (Eruvin 96a) that in every place that it states, "guard yourself," "lest," and "do not" (heeshamer, pen ve-al), it is certainly a negative commandment.