In the Mundaka Upanishad the sages say that there are two kinds of knowledge – the Higher Knowledge and the lower knowledge.
Knowledge that is concerned with rituals, grammer, etymology, metre, astronomy, etc. is considered the lower knowledge.
The Higher Knowledge is that by which the Imperishable Brahman is attained. The Brahman which is eternal, omnipresent, all-pervading and extremely subtle is beheld everywhere by means of the Higher Knowledge.
This Upanishad tries to make a clear distinction between performing sacrifices and humanitarian works on the one hand and pursuing the path of Higher Knowledge on the other to attain Brahman. Those who perform sacrifices and humanitarian works enter heaven but ultimately have to come back to earth and have to go through the endless cycles of birth and death. But a person whose mind is completely serene, whose senses are controlled, who has inner strong and earnestness, and is full of renunciation; such a person with the help of a wise teacher can realize the Brahman and become immortal.
In the Upanishad it is also stated that the resplendent and pure Atman, whom the sinless sannyasins behold residing within the body, is attained by unceasing practice of truthfulness, austerity, right knowledge, and continence. In fact the Atman cannot be attained through the study of the Vedas, or through intelligence, but one must choose the Atman then the Atman reveals its true nature to the seeker.
Two powerful analogies are used, one of bow, arrow and the target; and the other of two birds on a tree.
In the first analogy the syllable AUM is identified with the bow, the atman or the self to the arrow and the Brahman to the target. Through repeated practice the arrow is fixed into the target, the Brahmic Consciousness with the help of the bow. Through regular concentration, meditation, and contemplation, the seeker enters into the Absolute Consciousness of Brahman.
In the analogy of the two birds, one bird is seated on the top of the life-tree, the other on a branch below. The bird seated on the low branch eats both sweet and bitter fruits. Sweet fruits give the bird the feeling that life is pleasure; bitter fruits give the bird the feeling that life is misery. The other bird, seated on the top of the tree, eats neither the sweet fruit nor the bitter fruit. It just sits calmly and serenely. Its life is flooded with peace, light and delight. The bird that eats the sweet and bitter fruit on the tree of life is disappointed. It finds the life’s experiences impermanent, ephemeral, fleeting and destructive. Therefore this bird flies up and loses itself in the freedom-light and perfection-delight of the bird at the top of the life-tree. The bird on the top of the tree is the Cosmic and Transcendental Self, and the bird below is the individual self.