Mishnah Chagiga provides limitations on mystical inquiry, e.g., that the merkabah literature should not be attempted alone or by an unwise student. The tale of the four sages who glimpsed paradise, and especially Ahar, in Chagiga 14b-15b supplements this view.
But the reasoning here is obscure to me. As I read the story of Ahar (and Rubenstein's Talmudic Stories seems to agree), mystical speculation can lead to theological error, which leads to practical vice. In Ahar's case, this is apparently due to a fatalism born out of mystical speculation.
On the one hand, I understand how fatalism or a feeling of futility could lead to vice. So if mystical speculation inevitably leads to fatalism, then I can grok this passage. But on the other hand, I simply don't see why mystical inquiry is supposed to lead to this kind of anti-social fatalism. In the story, Ahar receives a very direct message that he has no share in ha-olam ha-ba'ah. I understand why he in particular feels like he might as well as sin.
But there's nothing to explain why the average person, who will not learn their fate in the world-to-come, should stay away from mystical speculation that exceeds their ability.
