The Isa Upanishad, in its first verse, takes us at once to the secret depths of Truth:

ॐ ईशा वास्यमिदं सर्वं यत्किञ्च जगत्यां जगत् ।

तेन त्यक्तेन भुञ्जीथा मा गृध: कस्यस्विद्धनम् ।।१।।

(Om Isavasyamidam sarvam yatkinca jagatyam jagat; Tena tyaktena bhunjithah ma grdhah kasyasviddhanam) 1

‘Whatever there is changeful in this ephemeral world, all that must be enveloped by the Lord. By this renunciation, support yourself. Do not covet the wealth of anyone.’

This is a very profound utterance, unequivocal, and yet ex­tremely simple. The whole universe, it tells us, is filled with the spirit of God. And our experience of the manifold, of the sense world, must be seen in the light of this abiding truth. A bubble rises on a sheet of water, plays for an instant on the surface, and disappears. Whence did it come, what was it, and where did it go? From water it came; having come, it is water still; and unto water it returns at the end.

In the language of Vedanta, there must be both a negation and an affirmation, if we are to enjoy this world. तेन त्यक्तेन भुञ्जीथा (Tena tyaktena bhunjitha), ‘by this renunciation, support yourself’, says this verse. What supports us is not what we renounce, but what we possess and enjoy; and this verse tells us to enjoy the world through possessing God. This world is worth enjoying, and we should enjoy it with zest. Zest in life is expounded throughout the Bhagavad Gita and the Upanishads. The great teachers who discovered these truths were not kill-joys; they were sweet and lovable men. Sri Ramakrishna was full of joy and Sri Krishna was full of joy. Jesus, too, was really a man of joy, although later dogma made him a man of sorrows. Before we can enjoy this world, however, we have to learn the technique of enjoyment. This technique is described in detail in the Bhagavad Gita, but here, in this first verse of the Isha Upanishad, the technique is summed up in that one word ‘renunciation’.

Renunciation is an eternal maxim in ethics as well as in spirit­uality. There is no true enjoyment except what is purified by re­nunciation. In our daily lives, in inter-personal relationships, we observe that we achieve the greatest joy not when we affirm our­selves, but when we deny ourselves.

Reference: The Message of the Upanishads by Swami Ranganathananda (p.75,76,77)