Geneivat Da'at
גניבת-דעת
The trait of honesty and truth must be the foundation of relations between a person and his fellow. The Torah therefore established that it is forbidden for a person to mislead his fellow into thinking that he did him a favor, when in fact he did nothing for him at all. This is the prohibition of geneivat da'at ("stealing the mind," i.e., deception).
What sets geneivat da'at apart from other lies is that a lie is generally directed at a particular event: instead of recounting accurately what happened, or what will happen, one lies and changes the facts. Geneivat da'at, however, is not directed at a particular event; its aim is more general – to change the natural attitude of one person toward another. Another difference is that a lie is carried out through speech, whereas geneivat da'at can also be carried out through behavior, such as flattery and the like.
To understand the severity of the prohibition of geneivat da'at, we must first note that every person has an opinion about the friends around him, an opinion based on the sum of the experiences they have shared in the past. In this way he knows which of his friends loves him more and which less, with whom it is fitting to be closer and with whom less so, and likewise, in a time of need, on whom he can rely more and on whom less.
But when a person presents himself as his fellow's great friend, ready to invest in him far beyond what is customary, and in practice all this pretense is only in order to gain benefits from their so-called friendship, he is stealing his fellow's mind. For in truth, his fellow should have thought of him in ordinary terms, yet because of his performances he regards him as one of his good friends.
The central problem is that when a person truly needs his good friends to stand by him and help him, he suddenly discovers that all sorts of people whom he considered his good friends vanish from the horizon, and in fact their entire friendship was one big act of geneivat da'at. In reality they wanted to derive benefits from their friendship with him, and therefore they deceived him and pretended to love him; but in a time of trouble, they slipped away one by one.
Worst of all, following such disappointments, people lose their trust in all human beings and in the value of friendship, wrap themselves in a shell of indifference toward society, and thereby lose the most precious human quality of all – the ability to open up and love others. As R. Akiva already said, the mitzva of "Love your fellow as yourself" (Vayikra 19:18) is a great principle of the Torah.
Now we can understand why the prohibition of geneivat da'at is so severe, to the point that Rabbeinu Yona wrote in his book Sha'arei Teshuva (Gate 3, 184) that in the eyes of the Sages the sin of geneivat da'at is more severe than robbery. For it strikes at the very core of a person's soul – his ability to develop sincere relationships of friendship and love with his fellows.