You shall not steal. You shall not deny falsely. You shall not lie, one man to his fellow. (Leviticus 19:11)
The Gemara describes certain types of deception as examples of theft (Chullin 94a). Rabbi Yom Tov ben Avraham Asevilli (d. 1330) explains that although the verse does not relate a specific type of theft, it is understood to include a range of behaviours which result in the perpetrator acquiring something that is not rightfully theirs (Ritva loc. cit.).
If someone behaves in a dishonest way which results in tricking others into thinking that they should be indebted or more grateful towards the perpetrator than he deserves, or gains unwarranted honour or favour, the act is considered geneivat daat (lit. stealing of the mind).
There are many practical examples of geneivat daat which may appear more innocent than they are. When a host decanters a cheap whisky into an empty bottle taken from a more expensive one, he deceives his guests into thinking that his generosity is greater than it is. Cheating in an exam is another example whereby a student gains praise for a false achievement. The sale of fake merchandise passed off as genuine or advertisements claiming discounted goods which have had their prices artificially inflated and then deflated are all examples of geneivat daat.
In line with this there are many examples of behaviour which are entirely legal (and often employed by salesmen and advertisers) but nonetheless violate the prohibition of geneivat daat. The concept of caveat emptor or ‘buyer beware’ is well known in secular law. It is the buyer’s responsibility to investigate the item they wish to purchase for imperfections or flaws. While the sale of goods act (1979) requires goods to be ‘fit for purpose’, cosmetic damage and the like could be legally concealed from a potential buyer. In Jewish Law the responsibility lies will the seller to inform the potential buyer of any defects or damage which might impact the value of the item.
Theft is not restricted to bank robbery and shoplifting. We must be honest in all of our dealings and resist taking anything that does not belong to us, whether material goods or honour and respect.