And this is what Baha'u'llah's sister says about him:
The Mirzā (meaning Baha’u’llah)...., from the beginning of his life to when he came of age—because the means were at hand and because of the gathering of the companions—was engrossed in studying and endeavored in homework. He wouldn’t disengage from learning the rudiments for a moment. After studying the rudiments of Arabic and literature he inclined towards the science of philosophy (ḥikmat) and mysticism (`irfān) so that he might benefit from these. It was such that he would spend most of the day and night socializing with high statured philosophers and the gatherings of mystics and Sufis. When it was blown in Seraph’s Trumpet of Appearance (meaning when the Bāb made his claims), he (meaning Baha’u’llah) was a man who had seen most of the words and phrases of the mystics and philosophers and had heard and understood most of the signs of the appearance (of the Mahdi) . . . after returning from Badasht and after the Shaykh Ṭabarsī Fort war was over, he was engaged day and night in socializing with great Islamic scholars and followers of mysticism . . .
-'Izziye Khānum (Khānum Buzurg), Tanbīh al-nā’imīn, pp. 4–5