SADHANA by Tagore
"Man's cry is to reach his fullest expression. It is this desire for self-expression that leads him to wealth and power. But he has to discover that accumulation is not realisation. It is the inner light that reveals him, not outer things. When this light is lighted, then in a moment he knows that Man's highest revelation is God's own revelation in him. And his cry is for this--the manifestation of his soul, which is the manifestation of God in his soul. Man becomes perfect man, he attains his fullest expression, when his soul realises itself in the Infinite being who is Avih whose very essence is expression." ~ Tagore

Sadhana is sanskrit term meaning spiritual practice or discipline. In yoga, sadhana is a practice to gain realization of self as a soul. Rabindranath Tagore is one of my all time favorite authors and the first non European to win the Nobel Prize in Literature in 1913. I highly recommend any and all of his writings... his prose, poetry, plays, songs... you will love them all! But first, get yourself a copy of SADHANA: The Realization of Life.
The Upanishads say with great emphasis, Know thou the One, the Soul. It is the bridge leading to the immortal being.
This is the ultimate end of man, to find the One which is in him; which is his truth, which is his soul; the key with which he opens the gate of the spiritual life, the heavenly kingdom. His desires are many and madly they run after the varied objects of the world, for therein they have their life and fulfilment. But that which is one in him is ever seeking for unity - unity in knowledge, unity in love, unity in purpose of will; its highest joy is when it reaches the infinite one within its eternal unity. Hence the saying of the Upanishad, Only those of tranquil minds, and none else, can attain abiding joy, by realising within their souls the Being who manifests one essence in a multiplicity of forms.
Excerpt: SADHANA The Realization of Life by Rabindranath Tagore