"God is guru, original guru. God gave lessons to Brahmā. Brahmā gave lessons to Nārada. Nārada gave lessons to Vyāsadeva. Vyāsadeva gave lessons to us. This is Vyāsadeva's contribution. And if you follow this disciplic succession, then you get perfect knowledge. Otherwise, if you speculate, then you are in darkness, tamasi. What power you have got? All your senses are imperfect. How you can get perfect knowledge? That is not possible. Therefore the injunction is, tad-vijñānārthaṁ sa gurum evābhigacchet (MU 1.2.12): 'You must go to guru.' And who is guru? This paramparā system. Evaṁ paramparā-prāptam imaṁ rājarṣayo viduḥ (BG 4.2).
So this is the process. If you want perfect knowledge, you must approach guru. And who is guru? Guru means the representative of the Supreme Personality of Godhead. Ācāryaṁ māṁ vijānīyān nāvamanyeta karhicit (SB 11.17.27). 'Ācārya,' Kṛṣṇa says, māṁ vijānīyāt, 'he is Myself. I am. Because he is My perfect representative—he won't speak anything nonsense; he will speak something or everything which he has heard from Me—therefore he is ācārya.' Ācārya means one who knows the śāstra and practically uses in his life, and the same thing, he teaches to his disciple. That is called ācārya. Ācārya is not a self-made man. No. Ācārya means ācinoti yaḥ śāstrāṇi. One who understand the śāstra, the Vedic śāstra, and practices in life and teaches the same thing to his student—that is called ācārya. So ācāryaṁ māṁ vijānīyān nāvamanyeta karhicit, na asūyeta martya-buddhyā (SB 11.17.27).
So ācārya should not be considered as ordinary man, because he is representative of Kṛṣṇa. Sākṣād-dharitvena samasta-śāstraiḥ. Viśvanātha Cakravartī Ṭhākura says, 'All the śāstras..."'The śāstra is the basic strength, platform. So all the śāstra says the ācārya: 'He is representative of the Supreme Lord.' So sākṣād-dharitvena. Sākṣāt: not indirectly, supposingly. No. Directly, sākṣāt. Sākṣāt means directly."
(Srila Prabhupada Lecture, Bombay, January 9, 1975)