Four rabbis entered the Pardes (פרדס)—the orchard, a code word for the deepest levels of mystical knowledge. According to Chagigah 14b, only one came out whole.
The four were Ben Azzai, Ben Zoma, Aher (Elisha ben Abuya), and Rabbi Akiva. Before they entered, Rabbi Akiva warned them: "When you arrive at the stones of pure marble, do not say 'Water, water,' for it is stated: 'He who speaks falsehood shall not be established before My eyes' (Psalms 101:7)."
Ben Azzai gazed at the divine and died. The verse applied to him: "Precious in the sight of the Lord is the death of His pious ones" (Psalms 116:15). Ben Zoma gazed and went mad. His verse: "Have you found honey? Eat only your fill, lest you become full of it and vomit it" (Proverbs 25:16). Aher—Elisha ben Abuya—gazed and "cut the saplings." He became a heretic, abandoning the faith entirely.
Only Rabbi Akiva entered in peace and departed in peace.
The narrative is surrounded by related accounts of mystical experiences. Rabban Yohanan ben Zakkai was riding a donkey while his student Rabbi Elazar ben Arakh asked to expound on the Design of the Divine Chariot—the Ma'aseh Merkavah (מעשה מרכבה). Yohanan ben Zakkai dismounted, wrapped his head, and sat on a stone. He explained: "The Divine Presence is with us. The ministering angels are accompanying us. I cannot ride a donkey during this."
Rabbi Elazar began to expound, and fire descended from heaven and encircled all the trees in the field. The trees broke into song: "Praise the Lord from the earth" (Psalms 148:7). An angel spoke from within the flames: "This is the very Design of the Divine Chariot." God's most dangerous knowledge, it turns out, is also the most beautiful.