Our Gemara on Amud Aleph discusses the prohibition of intentionally misleading another person. This is called geneivas daas, literally stealing knowledge. Although the Halacha has many details, the basic prohibition is when one intentionally misleads another person in order to profit. The profit does not have to be financial; it could be good will, such as the cases discussed in our Gemara, where a person was led to believe that an extraordinary honor or favor was done, when in fact the person did it for other reasons.

There is a remarkable Rabbenu Yonah (3:184) that states the following:

“One who fools his fellow, saying that he did him a favor or spoke favorably about him, but he did not do [so]. Our Rabbis, may their memory be blessed, said (Chullin 94a) [that] it is forbidden to deceive (literally, steal the thoughts of) people. And note that this was more weighty among the Sages of Israel than theft. For lying is certainly [a cause of] great guilt. And we have been obligated about maintaining careful boundaries when it comes to truth because [truth] is from the foundations of the soul.”

It is noteworthy that Rabbenu Yonah states that geneivas daas is worse than theft, and presumably that rationale is that it corrupts the soul in a foundational manner. To a spiritual person, the spiritual world is more real than the physical. Money, like other physical things, even life itself, is finite and comes and goes. However, ideas, thoughts, especially truth itself, is the way the soul connects to God and becomes eternal (see Rambam’s introduction to Perek Chelek). A corruption in truth is a corruption in connection to God.

From a psychological point of view as well, mental disorders often involve an overly defended ego. This means that there are feelings or ideas, especially conflicts, that are so difficult to face consciously, that they are repressed and denied. The person is not living in reality. Instead, they are altering reality to fit what feels better. This, on a small scale, happens to everybody. At times, denial is a healthy defense, such as, we don’t want to think too much about how we’re all going to end up dead. On the other hand, if a person is constantly denying what he is feeling and what he is thinking, because it is too painful to face, he will end up not managing reality. Because reality does not care about what you believe or what you feel; after all, it is simply reality. And when one does not live closely connected to reality, he will make mistakes, big and small. This is true in relationships, as well as professionally.

That is why the habit of lying is destructive and corrosive to the foundation of the soul.