Halakhah su Ecclesiaste 10:12

That we not believe in a god besides God alone: That we not believe in a god besides God, may He be blessed, alone, as it is stated (Exodus 20:3), "You shall have no other gods in front of Me." And its understanding is [that] you not believe in another god, except Me. And Ramban, may his memory be blessed wrote (Ramban on Exodus 20:3) [that] you will only find that Scripture states, "other gods," about belief of the heart. But concerning their making, it will never state, "do not make other gods," since the expression, "making," does not sit with others (since they cannot be made). And he, may his memory be blessed, was nicely precise [in this] - 'the words of the wise are grace!' And this commandment is the great fundamental principle of the Torah, as everything is dependent upon it - as they, may their memory be blessed, said (Sifrei Bamidbar 111:1), "Anyone who concedes to idolatry is as if he denied the whole entire Torah." And it is the same whether he accepts anything as a god besides only God, or whether he worships it according to its worship - meaning to say, in the way that those that believe in it worship it - or even not according to its worship, if he worships it according to the four well-known [forms of] worship, and they are sacrificing, bringing incense, pouring and bowing; he has transgressed upon "you shall have no." And pouring and sprinkling are one thing, and those that sprinkle are liable, [just] like those that pour.

From the laws of the commandment is that which they, may their memory be blessed, said (Sanhedrin 54a) that the prohibition of the mother is even if she was raped by his father, as behold she is his mother in any event. And so [too,] that which they said (Sanhedrin 53a) that one who has intercourse with his mother that is the wife of his father - meaning that she was not raped by him - is liable for two, on account of his mother and on account of the wife of his father. And there is no distinction in this whether he had intercourse in his father's lifetime or after his death, as she is always called the wife of his father, and [so] he is liable for two, as we said. And so [too,] that which our Rabbis, may their memory be blessed, forbade (Yevamot 21a) as a fence of this prohibition of the mother, the mother of his mother - meaning to say, his grandmother - and likewise the mother of his grandmother, going way back even to a thousand generations if it was possible to see them all; they are all forbidden by the words of the [Rabbis]. And so [too] they forbade the mother of his father - meaning to say, his grandmother from the father's side - as a fence for the mother, and likewise the mother of his grandmother, going way back. And so [too,] they forbade just the mother of the father of his father, and just the mother of the father of his mother only, as a fence of this [prohibition] of the mother. And there are some (Rashi on Yevamot 21a s.v. dalet nashim) that explained that these are also forbidden way back. And if you ask why the prohibition extends way back with the first ones - according to the first opinion - more than with these last two; the reason is because the first ones approach the prohibition of the mother which is the main prohibition, as behold they have a forbidden grandmother, which is the mother of the mother or the mother of the father. And therefore is it fitting to decree about them even going way back. But with these last two, they do not have a forbidden grandmother. And since the matter does not approach the mothers which is the main prohibition, they, may their memory be blessed saw (said) that it is enough that we decree with these two alone. And 'the words of the Sages are grace.' And these that are forbidden rabbinically are called secondary ones - meaning to say, they are secondary to those sexual prohibitions that were prohibited by the Torah.