The Mundaka Upanishad paints this spiritual journey of man from helplessness to fulfilment in a passage of surpassing poetic and spiritual charm (III. 1. 1-3)

द्वा सुपर्णा सयुजा सखाया समानं वृक्षं परिषस्वजाते ।

तयोरन्य: पिप्पलं स्वाद्वत्त्य नश्नन्नन्यो अभिचाकशीति ।।

(Dva suparna sayuja sakhaya samanam vriksham parishasvajate; Tayoranyah pippalam svadvattya nasnannanyo abhicakasiti.)

‘Two birds of beautiful plumage, knit in bonds of lasting friendship, live on the self-same tree. One of them eats the tasteful fruits of the tree, while the other, not eating, sits immersed in its own glory.’

समाने वृक्षे पुरुषो निमग्नो- ऽनीशया शोचति मुह्यमान: ।

जुष्टं यदा पश्यत्यन्यमीश- मस्य महिमानमिति वीतशोक: ।।

(Samane vrikshe purusho nimagno nishaya sochati muhyamanah; Jushtam yada pashyatyanyamisha- masya mahimanamiti vitasokah)

‘On the self-same tree (of life) is man immersed, helpless, he grieves, bound in delusion’s net; But when he perceives the other, the adorable, the Lord, all grief he casts off, knowing himself to be only the glory of this One.’

यदा पश्य: पश्यते रुक्मवर्णं कर्तारमीशं पुरुषं ब्रह्मयोनिम् । तदा विद्वान् पुण्यपापे विधूय निरञ्जन: परमं साम्यम् उपैति ।।

(Yada pashyah pashyate rukmavarnam kartaramisham purusham brahma-yonim; #### Tada vidvan punya-pape vidhuya niranjanah paramam samyam upaiti.)

‘When the wise seeker realises the effulgent Self, the Creator, the Lord, the source of Nature all, cleansed then of merit and demerit does the wise one become; and stainless, supreme oneness does he then achieve (with the Self of all).’

Expatiating on the message of these three verses, Swami Vivekananda says in his lecture on ‘Vedanta in Its Application to Indian Life’ ( CW. Vol. III, Mayavati Memorial Edition, pp. 235-36):

‘‘This is the picture of the human soul. Man is eating the sweet and bitter fruits of this life, pursuing gold, pursuing his senses, pursuing the vanities of life — hopelessly, madly careering he goes. In other places the Upanishads have compared the human soul to the charioteer, and the senses to the mad horses, unrestrained. Such is the career of men pursuing the vanities of life, children dreaming golden dreams only to find that they are but vain, and old men chewing the cud of their past deeds, and yet not knowing how to get out of this network. This is the world. Yet in the life of every one there come golden moments; in the midst of the deepest sorrows, nay, of the deepest joys, there come moments when a part of the cloud that hides the sunlight moves away, as it were, and we catch a glimpse, in spite of ourselves, of something beyond — away, away beyond the life of the senses; away, away beyond its vanities, its joys, and its sorrows; away, away beyond nature, or our imaginations of happiness here or hereafter; away beyond all thirst for gold, or for fame, or for name, or for posterity. Man stops for a moment at this glimpse, and sees the other bird calm and majestic, eating neither sweet nor bitter fruits, but im­mersed in his own glory, self-content, self-satisfied.…Man catches a glimpse, then again he forgets and goes on eating the sweet and bitter fruits of life; perhaps after a time he catches another glimpse, and the lower bird goes nearer and nearer to the higher bird as blows after blows are received. If he be fortunate to receive hard knocks, then he comes nearer and nearer to his companion, the other bird, his life, his friend; and as he approaches him, he finds that the light from the higher bird is playing round his own plum­age; and as he comes nearer and nearer, lo! the transformation is going on. The nearer and nearer he comes, he finds himself melt­ing away, as it were, until he has entirely disappeared. He did not really exist; it was but the reflection of the other bird, who was there calm and majestic amidst the moving leaves. It was all his glory, that upper bird’s. He then becomes fearless, perfectly satis­fied, calmly serene.’’

Reference: The Message of the Upanishads by Swami Ranganathananda (p.365,366,367)