Various Upanishads deal with the same topic in different ways. Thus one will find in Ishavasyopanishad, the direct discussion, while in some of the Upanishads, one will find an illustration by story depicting the philosophical discussion. For e.g., in Kathopanishad there is a story of Nachiketa having dialogue with the deity of death- Lord Yamaa about what happens after death.

The entire philosophy is based on the fundamental assumption viz., "existence exist" which was ultimately revealed to the seekers after the meditation and contemplation on the subject.The essence of existence being revealed was termed as "Brahman" from where the entire universe is emerged. The life force in every individual being is part of "Brahman" and is named as "soul" or "Aatmaa."

The individual soul when born and appear as an individual being, is bound by desires and this ultimately binds it to the Universe for life after life.

The purpose of each soul is to get rid of these bindings either by repayments of accumulated debts due to karma (past actions) or by burning it off with spiritual practice and come out of the cycle of birth and death in the universe and merge with the parent substance "Brahman" or God, if one prefers to use that word.

Thus Upanishads also deal with the methodology of uniting with the parent substance "Brahman" or God.