Sefer HaChinuch ספר החינוך

To not swear falsely: To not swear falsely, as it is stated (Leviticus 19:12), “And you shall not swear in My name falsely.” And they, may their memory be blessed, explained (Shevout 21a) that this verse warns about an oath of speech. And an oath of speech is what is stated in the Torah (Leviticus 5:4), “Or a soul that swears to express with the lips to do bad or to do good.” And it is divided into four parts — two of the future and two of the past, such as swearing on something that was done or not done, and on something that in the future he will do or will not do. And an oath of speech is only practiced with things that it is possible for a person to do, whether in the past or in the future. How is of the past? “I ate,” or “I did not eat”; and so [too,] “I threw,” or “I did not throw a stone into the sea.” And how is of the future? “I will eat,” or “I will not eat”; or “I will throw,” or “I will not throw.” But with things that have a prevention from the Torah, an oath of speech is not practiced. As an oath only rests upon an optional matter — that if he wants, he does it and if he wants, he does not do it — as it is stated, “to do bad or to do good.” But with any matter of a commandment, there is an obligation upon him to do it. Therefore an oath of speech does not rest upon him, whether in the past or in the future — in the case that he swears to perform a commandment, and he did not perform it; and so [too,] if he swears that he performed a commandment, and he did not perform it. As [just] like a liability [for punishment for a false oath] does not rest upon the matter of a commandment in the future, so too does it not rest upon it in the past. And so is the matter elucidated in its place in Shevuot 27a.

And from this reason that we said — that the liability of an oath only rests upon something that he is permitted to do — they, may their memory be blessed, exempted from an oath of speech anyone who swears to do bad to others, since he is commanded to not do bad to his fellow. And Rambam, may his memory be blessed, wrote (Mishneh Torah, Laws of Oaths 5:16) that it appears to him that he is lashed nonetheless, on account of [making] a vain oath. And one who swears to do bad to himself — even though he is not allowed — is liable on account of an oath of speech, if he does not do [the] bad. [If] he swears to do good to others in something that is in his hand to do, and he did not do [it], he is liable on account of an oath of speech. [These] and the rest of the many details of oaths and the matter of their annulment are all elucidated nicely in the tractate that is built on this, and that is Tractate Shevuot. And I have already written more than this regarding this commandment — and I wrote at length — on the commandment of “You shall not take” in the Order of Vayishma Yitro (Sefer HaChinukh 30).

And this prohibition is practiced in every place and at all times by males and females. And one who transgresses it — if he was volitional — is lashed, and specifically with witnesses and a warning, as is well-known in all of the commandments. And if he was inadvertent, he is obligated to bring a sliding-scale sacrifice. And so did they say in Shevuot 27b, “This is an oath of speech, which we are liable for lashes for its volitional transgression, and a sliding-scale sacrifice for its inadvertent transgression.” And that which they said, “This is” has no novelty other than from the side of the sacrifice, which is a sliding-scale offering.