The Upanisads in line with the divine Vedas propound the doctrine that all that exists is spirit or soul. The spiritualistic lesson innate in the Upanisads can be a panacea for all ills. Our great Rishis quoted from the holy Vedas to produce the Upanisads to show to the people the path of peace, happiness and freedom.
Isavasya or Isa-Upanisad is the fortieth and the last chapter of Yajurveda. This is why it is known as Vedant ---the knowledge that is available at the end of the Vedas. The Isa-Upanisad begins with the statement that the entire universe is enveloped by the Supreme Being and derives its substance from Him.
In the first mantra, the words used are "tayakten bhunjeetha". It means we should develop in us the capability enough to enjoy the world which has been made for us. But the world "bhunjeetha" is preceded by a word 'tayakten" which read together means that we should enjoy the world but with a sense of renunciation.
It is not a sin to earn, save and spend for self but one should not be selfish. Your wealth should be shared with those who are less fortunate and needy. Money is an important tool to alleviate the miseries of the poor and to uplift the society, but we should not develop attachment with it.
Another important message of Isa-Upoanisad is: "Have a will to live for full hundred years and lead a purposeful life by performing your duties". There is a purpose in creating man by God. By creating him, God expects him to perform, to remain firm, to endeavour, to struggle and to face perplexities and move ahead on the path of progress.
God has given man a precious life and has blessed it with all the needs for a comfortable existence.
The Isa does not suggest that the world should be renounced. But we are not to be self-centered; and should be in the pursuit of truth.