Ramban, on Exod 20:7

LO THISA’ (THOU SHALT NOT TAKE) THE NAME OF THE ETERNAL THY G-D IN VAIN. This verse has already been explained in the words of our Rabbis. He prohibits [here] swearing by the Glorious Name in vain, such as swearing that which is contrary to facts known to man, or swearing to [the truth of] a self-evident fact. For example: if one swears that a pillar of marble is of gold, or that it is of marble, and the pillar is right before them and they recognize it as such [that it is of marble]. By way of the plain meaning of Scripture, the verse also prohibits the taking of the Glorious Name in vain upon one’s lips [even without an oath], the usage of the term [lo thisa] being similar to these expressions: ‘Lo thisa’ (Thou shalt not utter) a false report; Nor ‘esa’ (do I take) their names upon my lips. Speaking is called thisa, [which literally means “lifting”], because the speaker thereby lifts up his voice. Similarly: ‘masa’ (The burden) of the word of the Eternal; also, In that day ‘yisa’ (shall he swear), saying: I will not be a healer, which means that he will lift up his voice to say so. And in truth, this — [i.e., just taking G-d’s Name in vain even without an oath] — is also forbidden, and in the language of the Sages, it is called “pronouncing the Name of Heaven to no purpose.” Thus our Rabbis have already said: 2. “Whence do we know that [in dedicating a beast for a sacrifice] a man should not say, ‘Unto the Eternal this is a whole-offering,’ or ‘Unto the Eternal this is a sin-offering,’ but instead he should say, ‘This is a whole-offering unto the Eternal,’ ‘This is a sin-offering unto the Eternal’? Scripture therefore says, an offering unto the Eternal. And must we not reason by using the method of kal vachomer?, Note 208. If the Torah said of him who is about to dedicate [something to Heaven], ‘Let My Name not rest on it until [he has first said] korban (sacrifice)’, is it not logical [that we must not pronounce the Name of Heaven to no purpose]!” He has placed this commandment after the prohibition of idolatry, because just as it is proper to fear the Great and Fearful Name by not giving His Glory to another, so it is fitting to give glory to His Name. He who takes it in vain profanes it, similar to that which is written, And ye shall not swear by My Name falsely, so that thou profane not the Name of thy G-d. Just as He was stringent in the case of idolatry and wrote the punishment [for transgression], i.e., that He is a jealous G-d, visiting the iniquity of the fathers upon the children, so did He record here the punishment that He will not hold him guiltless. He used this expression instead of saying that He will visit his sin upon him, [as He did in the case of idolatry], because people who swear [in vain] do not consider it a real sin, and they think it is proper that He forgive them. Therefore He said that whosoever toucheth that shall not go unpunished. Rabbi Abraham ibn Ezra has written appropriately on this verse. Now the language of this verse, the Name of the Eternal thy G-d, implies that it is as if Moses was speaking, and so also in the case of all the following commandments, whereas in the first two verses G-d is speaking: I; Who brought thee out; before Me; For I; Of them that love Me and keep My commandments. It is for this reason that our Rabbis of blessed memory have said: “We heard the two commandments — I am the Eternal thy G-d and Thou shalt have no other gods — from the Almighty Himself,” for they are the root of everything. But Rabbi Abraham ibn Ezra asked [concerning this tradition of the Rabbis] that Scripture says, And G-d spoke all these words, and still more clearly it is written [following the Ten Commandments], These words the Eternal spoke unto all your assembly, and again it is written there, And He wrote them down upon two Tablets of stone,, 4:13. meaning that as He said the Ten Commandments to all your assembly, so He wrote them down upon the Tablets! I will explain to you the tradition of our Rabbis [that we heard the first two commandments from the Almighty Himself]. Surely all Israel heard the entire Ten Commandments from the mouth of G-d, as the literal meaning of Scripture indicates. But in the first two commandments, they heard the utterance of speech and understood their words even as Moses understood them. Therefore He spoke to them directly [in the first person], just as a master speaks to his servant, as I have mentioned. From then on, in the rest of the commandments, they heard a voice of speech but they did not understand it, and it became necessary for Moses to explain to them each and every commandment until they understood it from Moses. And so [the Rabbis] explained:. In explanation thereof, the Sages said in the Mechilta that Moses brought to them every commandment as he heard it.” I have not been able to identify the exact quotation in the Mechilta. Moses spoke, and G-d answered him by a voice. Therefore [the rest of the Ten Commandments] were addressed by G-d to Moses so that he should tell them thus. The reason [that the first two commandments were spoken to the people directly by G-d] was so that they should all be prophets in the belief of G-d, [His existence, and His Unity], and in the prohibition of idolatry, as I have explained., 19:9. Those are the root of the whole Torah and the commandments, just as He said, Assemble Me the people, and I will make them hear My words, that they may learn to fear Me all the days. But in the rest of the Ten Commandments, they received their explanation from the mouth of Moses after having heard a voice of words,, Verse 12. while in all other commandments [of the whole Torah], they believed in Moses completely.