The professor has an ability to reduce san loss by 1. Therefore, he is able to cast any number of spells for free! Is this meant to be so overpowering?
Page 4 on Arkham Horror FAQ from the Fantasy Flight website:
Q: Does the Strong Mind ability (reduce all Sanity loss by 1) of the professor, Harvey Walters, apply to the casting of spells?
A: No. The professor’s ability works on losses, not costs. The same distinction applies to the gangster, Michael McGlen, and his ability to reduce Stamina losses.
According to the Arkham Horror Wiki, a later expansion clarified that the ability does not apply to spells or any other voluntary san loss, only involuntary. Therefore, he's not actually meant to be so powerful. If there's errata for the base game, I couldn't find any.