Vivekachudamani 3

दुर्लभं त्रयमेवैतद्देवानुग्रहहेतुकम् । 
मनुष्यत्वं मुमुक्षुत्वं महापुरुषसंश्रयः ॥ 
durlabhaṃ trayamevaitaddevānugrahahetukam | 
manuṣyatvaṃ mumukṣutvaṃ mahāpuruṣasaṃśrayaḥ || 

Very rare indeed are these three things and happen only due to the utmost Grace of God—a human birth, a burning desire for liberation, and the blessed refuge of an illuminated sage. The path of spiritual realization is undoubtedly difficult. The Lord, therefore, advises us to approach a bona fide spiritual master in the line of disciplic succession from the Lord Himself. The Bhagavatam says: dharmam hi saksad-bhagavat-pranitam—the path of religion is directly enunciated by the Lord. Therefore, mental speculation or dry arguments cannot help one progress in spiritual life. One has to approach a bona fide spiritual master to receive the knowledge towards liberation.