While it goes without saying that telling an outright lie is forbidden, even passive lying – omitting the entire truth or otherwise misleading someone – is forbidden. This is known as geneivat da’at, which means “stealing another person’s thoughts.” The principles of geneivat da’at teach that one must be honest and always speak in a manner that reflects one’s true thoughts and intentions. This applies to all human beings, whether Jewish or Gentile. Our sages teach that geneivat da’at is the worst form of theft that exists and is far worse than stealing property. We are taught the God hates those who deceive others.
- It is forbidden to trick or deceive someone in any manner. It is equally forbidden to trick a non-Jew.
- If an item one is selling has a flaw one must let the buyer know.
- It is forbidden to invite someone to an occasion if one knows that they can't attend because that is deceiving them that one intended to honor them and in reality one has no intention that would happen.
- Many poskim hold it is permitted to invite someone who you know can't come as long as you don't urge them excessively.
- One should not offer someone gifts knowing that he isn't going to take them because it is deceitful.
- When giving a gift it isn't considered deceitful to give them something of a lower quality even though they will assume it is a better quality.
Letting Someone Fool Themselves
- If someone makes an assumption that a rational person wouldn't make one doesn't need to tell them otherwise since they deceived themselves. For example, Mar Zutra was leaving his city and he met two rabbis. He assumed that they were coming to greet him and so he thanked them very much for coming. In fact they were traveling to that city anyway and weren't planning on going to greet Mar Zutra. Therefore, the gemara concludes that they didn't need to tell him that in fact they didn't come to greet him. Mar Zutra fooled himself and it wasn't their actions that were deceitful.
Cheating on Tests
- It is absolutely forbidden to cheat on a test since doing so is dishonest and deceives the one grading the test and the one receiving the scores. In fact, the halachic authorities express shock at the widespread disregard for cheating on tests, deeming it a sin equal to all others in the realm of lying, stealing, and cheating.
Increasing the Price of an Honor
- Many shuls commonly have auctions to buy a certain honor such as an aliyah or peticha and the funds go to the shul. If a person has no interest in buying the honor but simply bids in order to increase the price of the others who are bidding, even if they will actually pay for it if they win, it is a violation of being deceitful.
- Is one allowed to publicly pledge a donation to a tzedaka cause so that others give if one is actually going to give less? That is deceitful and forbidden. 
- It is forbidden to ask the price of a product if one has no intention of buying it.
- It is forbidden even to window shop if someone has no intention of buying it. Some explain that it is forbidden because it is insulting to the seller to think that someone is going to buy and then be turned down. Some explain that it is wasting the time of the seller. Some explain that the reason is because if they see that you want to buy it and then walk away they might lower their price and others hearing this will only buy it for that lower price. Some say that it will prevent a potential customer from buying it since they see someone else is trying to buy it.
- Someone who wants to buy a product certainly can go to a number of stores and ask their prices and then go back to the store that has the best price. That is normal practice and not deceitful since if the store that one is inquiring regarding the price would be best he would buy there.
- A competitor who wants to find out what other store sell their products for according to some poskim may be technically permitted to do so even though he has no intention on buying.
Advertising a Product that isn't for Sale
- It is forbidden to advertise to sell a product and then not sell it since that deceives buyers into coming and checking out that product.
- Someone who is selling his house and his neighbor wants should sell to him if the neighbor will pay the price that the seller could have gotten anyway. If the neighbor has a cap and says that they won't buy it above a certain price then the seller can put the house on the market and see what people are willing to pay for it. He isn't deceiving anyone since they have the chance to buy it if they are willing to pay more than the neighbor is willing to pay. However, if the neighbor doesn't have a cap it isn't right to put it on the market since doing so deceives the buyers into thinking that they have a chance to buy it and they don't.
- Rambam, Hilchot Deot 2:6; Rambam, Hilchot Mechira 18:1; Sefer Chassidim 51.
- Tosefta, Bava Kamma 7:3; Sha’arei Teshuva 3:184.
- Bava Metzia 58b.
- Pesachim 113b.
- In Chullin 94a Shmuel states that it is forbidden to 'steal the mind' of anyone including a non-Jew. This is codified by Shulchan Aruch CM 228:6
- Shulchan Aruch CM 228:6
- Shulchan Aruch CM 228:6
- Article on jlaw.com citing Bach, Beer Hagolah, and Sama
- Shulchan Aruch CM 228:6
- Tur 117:1 quotes a dispute whether or not the issue of genivat daat applies to a present. Rama YD 117:1 and Shach YD 117 write that we hold that there is only a prohibition of genivat daat when selling something for one quality and they assume it is better than it is. However, when giving a gift even though they'll assume it is a better quality than it is that isn't genivat daat.
- Gemara Chullin 94b, Shulchan Aruch CM 228:6
- Igrot Moshe 2:30, Mishneh Halachot 7:275
- Shevet Halevi 10:163.
- Chida in Yosef Ometz 57 writes that once there was a person who made a deal with the Gabbay. He would bid to increase the price for everyone else but if he won and was stuck with the honor he would only pay half. The Chida writes that this practice is forbidden for three reasons. 1) He is annoying and bothering those who really do need or want that honor and now have to pay more. 2) He is deceiving people in showing that he wants that honor and really doesn't. 3) He is lying since he bid to pay a certain amount and in fact he is paying half. Also, the Chida shows that the motivation of this person isn't sufficient to allow his deceitful behavior. He proves from Bava Batra that we don't even allow a Gabbay of tzedaka to force people to give tzedaka. Here the person isn't a Gabbay tzedaka and he is forcing people to give tzedaka. Additionally, that was only for tzedaka to the poor and not funds that just go to the shul. Article on jlaw.com generalizes this based on the first two reasons of the Chida.
- Minchat Yitzchak 3:97 writes that one may not pledge to give more tzedaka that one is actually going to since doing so is deceitful. He proves it from the Gemara Sukkah 29a which says that because people pledged to tzedaka and didn't give it Hashem punished them and their wealth was stolen by the government. The Maharsha adds that this is true even though those that pledged did so in order to cause others to give. He also proves it from Yerushalmi Brachot 7:2 in a story about Shimon Ben Shetach.
- Shulchan Aruch CM 228:4
- Shitah Mikubeset 58b citing Raavad s.v. lo yitleh, Pitchei Choshen Geniva ch. 5 fnt. 15
- Pitchei Choshen Geniva ch. 5 fnt. 15 citing Chovat Hashemira
- Pitchei Choshen Geniva ch. 5 fnt. 15 citing Meiri
- Pitchei Choshen Geniva ch. 5 fnt. 15 citing Rashbam Pesachim 112b
- Pitchei Choshen Geniva ch. 5 fnt. 15 writes that it is obvious that if one actually has an intention to buy some product but one wants to shop around for the best price that it is permitted to do so because there is a chance with any store that he will buy there.
- BusinessHalacha.com writes that according to the Shulchan Aruch Harav n. 28 it is only forbidden to inquire for a price when one has no intention of buying it if the seller will come to the conclusion that he was doing so to trick him. Therefore, in a case when one is window shopping it is only forbidden if the seller knows that one has no intention of buying it and is therefore annoyed. However, if they don't know they assume that like any other potential buyer they might or might not buy here and so they won't feel annoyed if someone asks for a price and doesn't buy it. [However, according to Pitchei Choshen's reason of not wasting the seller's time this seems forbidden.]
- Pitchei Choshen Geniva ch. 5 fnt. 15 citing Sefer Chasidim 311
- Pitchei Choshen Geniva ch. 5 fnt. 15