According to tradition, the Buddha’s dying words were (freely translated): “all compounds are perishable.”
It was not man’s immortal spirit he meant. Rather, Buddha was pointing to the temporary physical, passionate, personal and psychic parts of us.
Our deathless spirit uses sensory and physical vehicles merely as instruments of expression in every new ly minted life.
Functionally linked those parts are temporary and are separated from each other at death. They are reduced to their primal elements, like the fuel of a fire, recycled and returned back to Universal Nature, their primal state.
The process is entirely natural, the recycling of renewable substances of evolution, of the temporal forces and materials required for an earthly body:
“Dust thou art,” states Genesis (3:19), “and unto dust thou shalt return”— referring to those perishable parts of man’s complex construction.
By contrast, in a dream, “the Spirit of man is free,” as the occult teaching of the Brihad Aranyaka Upanishad (13) declared, “and naught adheres to the Spirit.”

Death is Sleep
Mme. Blavatsky agreed with the Upanishads, and explains in the Key to Theosophy (109) that “death ever comes to our spiritual selves as a deliverer and friend.” And for the average mortal, “it will be a dream as vivid as life, and full of realistic bliss and visions.”
Even for the materialist, who, “notwithstanding his materialism, was not a bad man, the interval between the two lives will be like the unbroken and placid sleep of a child,” Blavatsky wrote.
“As the man at the moment of death has a retrospective insight into the life he has led, so, at the moment he is reborn on to earth. He has a prospective vision of the life which awaits him and realizes all the causes that have led to it.
“He realizes them and sees futurity, because it is between [the bliss filled after-death dream state called] Devachan and re-birth, that the Ego regains his full [spiritual mind] manasic consciousness, and re-becomes for a short time the god he was —before he first descended into matter and incarnated in flesh, in compliance with Karmic law.”
“The ‘golden thread’ sees all its ‘pearls’ and misses not one of them.” – H. P. Blavatsky
“I repeat it: death is sleep. After death, before the spiritual eyes of the soul begins a performance according to a programme learnt and very often unconsciously composed by ourselves: the practical carrying out of correct beliefs or of illusions which have been created by ourselves. The Methodist will be Methodist, the Mussulman a Mussulman, at least for some time — in a perfect fool’s paradise of each man’s creation and making.

Fools Paradise
“These are the post-mortem fruits of the tree of life. Naturally, our belief or unbelief in the fact of conscious immortality is unable to influence the unconditioned reality of the fact itself, once that it exists; but the belief or unbelief in that immortality as the property of independent or separate entities, cannot fail to give colour to that fact in its application to each of these entities. Now do you begin to understand it?”
(H. P. Blavatsky, The Key to Theosophy, Section 9,
“What is Really Meant by Annihilation)