A seer of the Upanishads, after having followed the spiritual path and after having realized the supreme Spirit, spoke in these terms: "The wise ones say that the path to the realization of the supreme Spirit is as difficult to tread as the sharp edge a razor."

क्षुरस्य धारा निशिता दुरत्यया दुर्ग पथस्तत् कवयो वदन्ति ।

Katha Upanishad, 1.3.14

However, through proper training even the most difficult path may be followed and the supreme goal attained. This is what the ancient Hindu philosophical and spiritual teachers discovered.

What kind of training are we expected to undergo before we embark on the difficult but most important journey into the realm of the Spirit? To quote the Upanishad again: "Know the Atman to be the master of the chariot, and the body as the chariot. Consider the intellect (buddhi) as the charioteer, and the mind as the rein. The senses, they say, are the horses and the roads are the sense objects. The wise call Him (the Atman) the enjoyer when He is united with the body, the senses and the mind.

One who is always of an unrestrained mind, devoid of discrimination, has his senses uncontrollable like the wicked horses of the charioteer. But he who is always of a restrained mind, has right discrimination, has his senses controllable like the good horses of the charioteer. He who is devoid of right discrimination, who is thoughtless and always impure, never attains the goal and gets into the round of births and deaths. But he who is associated with a discriminating intellect, ever pure, with the mind controlled, reaches that goal whence none is again. The man who has a discriminating intellect for his charioteer and a well-controlled mind as the reins, attains the end of the road — the supreme experience of the all-pervading Spirit.

Reference: Meditation and Spiritual Life by Swami Yatiswarananda (p.107)