Little deceptions such as inviting people to dinner when you know that they already have a previous engagement or pretending that you remembered someone’s birthday when you really didn’t are often thought to be harmless “white lies”. The Torah, however, forbids them under the category of gneivat da’at, literally “mind theft”. Gneivat da’at involves generating gain, or potential gain (eg through creating a sense of obligation on another’s part) by means of a falsehood.
According to Rabbi Yonah Girondi (13th century), gneivat da’at is even worse that outright robbery because it corrupts our capacity to develop honest and sincere relationships. But if a friend wrongly presumes that you’ve called to wish him/her happy birthday, you do not have to fess up. The Talmud (Chulin 94b) reasons that if people jump to the wrong conclusion, through no fault of yours, you should not disabuse them if it means offending them.
The rules of gneivat da’at epitomise the values of truth and respect for others. Smooth talk may seem useful but it quickly corrodes our character.