SB 1.9.16

na hy asya karhicid rājan
 pumān veda vidhitsitam
yad vijijñāsayā yuktā
 muhyanti kavayo ’pi hi
Word for word: 
na — never; hi — certainly; asya — His; karhicit — whatsoever; rājan — O King; pumān — anyone; veda — knows; vidhitsitam — plan; yat — which; vijijñāsayā — with exhaustive inquiries; yuktāḥ — being engaged; muhyanti — bewildered; kavayaḥ — great philosophers; api — even; hi — certainly.
Translation: 
O King, no one can know the plan of the Lord [Śrī Kṛṣṇa]. Even though great philosophers inquire exhaustively, they are bewildered.
Purport: 

The bewilderment of Mahārāja Yudhiṣṭhira over his past sinful acts and the resultant sufferings, etc., is completely negated by the great authority Bhīṣma (one of the twelve authorized persons). Bhīṣma wanted to impress upon Mahārāja Yudhiṣṭhira that since time immemorial no one, including such demigods as Śiva and Brahmā, could ascertain the real plan of the Lord. So what can we understand about it? It is useless also to inquire about it. Even the exhaustive philosophical inquiries of sages cannot ascertain the plan of the Lord. The best policy is simply to abide by the orders of the Lord without argument. The sufferings of the Pāṇḍavas were never due to their past deeds. The Lord had to execute the plan of establishing the kingdom of virtue, and therefore His own devotees suffered temporarily in order to establish the conquest of virtue. Bhīṣmadeva was certainly satisfied by seeing the triumph of virtue, and he was glad to see King Yudhiṣṭhira on the throne, although he himself fought against him. Even a great fighter like Bhīṣma could not win the Battle of Kurukṣetra because the Lord wanted to show that vice cannot conquer virtue, regardless of who tries to execute it. Bhīṣmadeva was a great devotee of the Lord, but he chose to fight against the Pāṇḍavas by the will of the Lord because the Lord wanted to show that a fighter like Bhīṣma cannot win on the wrong side.