Q. What is the Jewish opinion on hypnosis?
On the first issue, the answer depends on whether hypnosis is merely a resort to dark supernatural practices, which would presumably come under the heading of witchcraft.
There was a time when hypnosis was rare and treated with suspicion by the medical profession, but now that it has some legitimacy we would tend to follow the view of the 19th-century authority, Rabbi Jacob Ettlinger, in responsum no. 67 in his Binyan Tziyyon. He was asked about a pious man who was advised to undergo magnetism or mesmerism. Gentile physicians assured the rabbi that the procedure had mental health benefits and was not necromancy. The rabbi therefore approved hypnotism as a valid treatment.
In relation to the second problem, Rabbi Joseph Schwarz addresses the question of a hypnotised shochet (kosher animal slaughterer) in the magazine Vay’lakket Yosef. If the shochet carries out shechitah whilst under hypnosis, is his act halachically valid?
Technically he does not need the same degree of mental intention as in saying the Shema, where mental intention is everything. Nonetheless, if the hypnotised shochet carries out the act properly he must have some level of mental awareness and his act is valid. The discussion is reported by Solomon B. Freehof in his “Responsa Literature”, 1959, ch. 7.