The Tosefta in Masechet Bava Batra lists seven categories of theft, and mentions at the top of the list, as the most grievous form of thievery, the sin of "Geneivat Da'at," which literally means "theft of the mind," referring to deception. The Ritva (Rabbi Yom Tov Ashbili, Spain, 1250-1330), in his commentary to Masechet Chulin, writes that deceiving another person transgresses a Torah violation. According to the Ritva, the verse "Lo Tignovu" ("Do not steal" – Vayikra 19:11) refers specifically to this form of "theft," and thus a person who deceives another violates this Torah prohibition. This is also the position of the Yerei'im (by Rabbi Eliezer of Metz, France, 1115-1198), in Siman 124. Others, however, maintain that deception transgresses a Rabbinic edict, and not a Torah violation.
We present here a number of practical examples of Geneivat Da'at.
The Gemara mentions the example of somebody who invites his fellow to join him for a holiday, fully aware that this person had already made plans to spend the holiday elsewhere. He deceives his fellow into thinking that he sincerely wishes to host him, whereas in reality he has no actual intention to extend the invitation. Even though no practical harm results from this disingenuous invitation, it nevertheless violates the prohibition of Geneivat Da'at, insofar as the individual has deceived his fellow.
In the context of business operation, the issue of Geneivat Da'at arises quite frequently. For example, a proprietor may not announce a 50% price reduction and then raise the prices so that the sale price will amount to the item's actual price. Even though he in the end receives a fair price for the merchandise, he has nevertheless transgressed the prohibition of Geneivat Da'at because he has deceived the consumers.
The Poskim (Halachic authorities) discuss the question of whether a person who purchased a gift for his fellow at a discount price may leave the original price-tag on the package, so that the recipient will think he paid the price listed on the tag. Would this be considered Geneivat Da'at, in that the giver deceives the recipient into thinking that he paid a higher price for the gift? The Poskim generally conclude that one may leave the price-tag on the package, since the price on the tag is, after all, the true value of the item purchased.
Another question addressed by the Poskim concerns a case of one who wishes to give a gift. For example, it is forbidden to gift a set of Zohar that has several pages missing. The person giving the gift assumes that the recipient will never actually study the Zohar and will thus never realize that the set is missing pages. The Poskim write that it is forbidden to give this set as a gift, since one thereby deceives the recipient into thinking that he has received a perfect set of Zohar, whereas in reality it is defective.
Rabbi Moshe Feinstein (Russia-New York, 1895-1986) records in his work Iggerot Moshe a question he was asked as to whether Yeshiva High School students may be shown the answers to the Regents' Exams before the tests, so that they will not have to study the material and could use the extra time for Torah study. Rabbi Feinstein responds by expressing his astonishment over the very posing of such a question. Cheating is a clear violation of Geneivat Da'at, and it cannot possibly be allowed even for the purpose of facilitating additional time for Torah study.
Rabbenu Yona of Gerona, Spain (1180-1263), in his work Sha'arei Teshuva, compares one who steals property committing a sin with his hands to sins involving deceit. A person's soul, he explains, is a part of the Almighty Himself, the embodiment of perfect, unadulterated truth. Engaging in deceitful conduct contaminates this Godly quality of the soul, and thus constitutes a most grievous sin, more severe than the theft of property.
Summary: Geneivat Da'at, deception, is deemed by Halacha as the most grievous form of theft. One may not extend an invitation to his fellow knowing full well that he already has plans, so as to give the impression of extending a sincere invitation. Likewise, a retailer may not announce a price reduction and then raise the price so that the sale price will amount to the actual price. It is also forbidden to give a defective gift even if the recipient will never notice the defect. And it is certainly forbidden to cheat on exams, even for the purpose of allowing more time for Torah study. However, one who purchases a gift at a discounted price may leave the price-tag on the package, even though the price tag lists the item's price before the discount.