The Following, from Chapter 6 of Isis Unveiled I, is one of Blavatsky's first discussions of the mysteries of the astral plane and serves as a good introduction to the topic. Chapters 5-8 Of Isis I are a rich source of information on this vast and complex subject.
The Astral Light is a storehouse record of all thoughts and deeds on earth
"The oracles assert that the impression of thoughts, characters, men, and other divine visions, appear in the aether. . . . In this the things without figure are figured," says an ancient fragment of the Chaldean Oracles of Zoroaster (Simpl. in Phys.," 143; "The Chaldean Oracles," Cory).
Thus, ancient as well as modern wisdom, vaticination and science, agree in corroborating the claims of the kabalists. It is on the indestructible tablets of the astral light that is stamped the impression of every thought we think, and every act we perform; and that future events — effects of long-forgotten causes — are already delineated as a vivid picture for the eye of the seer and prophet to follow. Memory — the despair of the materialist, the enigma of the psychologist, the sphinx of science — is to the student of old philosophies merely a name to express that power which man unconsciously exerts, and shares with many of the inferior animals — to look with inner sight into the astral light, and there behold the images of past sensations and incidents. Instead of searching the cerebral ganglia for "micrographs of the living and the dead, of scenes that we have visited, of incidents in which we have borne a part,"(Draper: "Conflict between Religion and Science.") they went to the vast repository where the records of every man's life as well as every pulsation of the visible cosmos are stored up for all Eternity!
Retrospective memory of all our life in near-death experiences
That flash of memory which is traditionally supposed to show a drowning man every long-forgotten scene of his mortal life — as the landscape is revealed to the traveller by intermittent flashes of lightning — is simply the sudden glimpse which the struggling soul gets into the silent galleries where his history is depicted in imperishable colors.
Experiences of déjà vu in situations and places
The well-known fact — one corroborated by the personal experience of nine persons out of ten — that we often recognize as familiar to us, scenes, and landscapes, and conversations, which we see or hear for the first time, and sometimes in countries never visited before, is a result of the same causes. Believers in reincarnation adduce this as an additional proof of our antecedent existence in other bodies. This recognition of men, countries, and things that we have never seen, is attributed by them to flashes of soul-memory of anterior experiences. But the men of old, in common with mediaeval philosophers, firmly held to a contrary opinion.
The astral body is set free during sleep
They affirmed that though this psychological phenomenon was one of the greatest arguments in favor of immortality and the soul's preexistence, yet the latter being endowed with an individual memory apart from that of our physical brain, it is no proof of reincarnation. As Eliphas Levi beautifully expresses it, "nature shuts the door after everything that passes, and pushes life onward" in more perfected forms. The chrysalis becomes a butterfly; the latter can never become again a grub. In the stillness of the night-hours, when our bodily senses are fast locked in the fetters of sleep, and our elementary body rests, the astral form becomes free. It then oozes out of its earthly prison, and as Paracelsus has it — "confabulates with the outward world," and travels round the visible as well as the invisible worlds. "In sleep," he says, "the astral body (soul) is in freer motion; then it soars to its parents, and holds converse with the stars."
Effects of our astral sleep life
Dreams, forebodings, prescience, prognostications and presentiments are impressions left by our astral spirit on our brain, which receives them more or less distinctly, according to the proportion of blood with which it is supplied during the hours of sleep. The more the body is exhausted, the freer is the spiritual man, and the more vivid the impressions of our soul's memory. In heavy and robust sleep, dream- less and uninterrupted, upon awakening to outward consciousness, men may sometimes remember nothing. But the impressions of scenes and landscapes which the astral body saw in its peregrinations are still there, though lying latent under the pressure of matter. They may be awakened at any moment, and then, during such flashes of man's inner memory, there is an instantaneous interchange of energies between the visible and the invisible universes.
Body-Mind connection during sleep
Between the "micrographs" of the cerebral ganglia and the photo-scenographic galleries of the astral light, a current is established. And a man who knows that he has never visited in body, nor seen the landscape and person that he recognizes may well assert that still has he seen and knows them, for the acquaintance was formed while travelling in "spirit." To this the physiologists can have but one objection. They will answer that in natural sleep — perfect and deep, "half of our nature which is volitional is in the condition of inertia"; hence unable to travel; the more so as the existence of any such individual astral body or soul is considered by them little else than a poetical myth.
Blumenbach assures us that in the state of sleep, all intercourse between mind and body is suspended; an assertion which is denied by Dr. Richardson, F. R. S., who honestly reminds the German scientist that "the precise limits and connections of mind and body being unknown" it is more than should be said. This confession, added to those of the French physiologist, Fournie, and the still more recent one of Dr. Allchin, an eminent London physician, who frankly avowed, in an address to students, that "of all scientific pursuits which practically concern the community, there is none perhaps which rests upon so uncertain and insecure a basis as medicine," gives us a certain right to offset the hypotheses of ancient scientists against those of the modern ones.
No man, however gross and material he may be, can avoid leading a double existence; one in the visible universe, the other in the invisible. The life-principle which animates his physical frame is chiefly in the astral body; and while the more animal portions of him rest, the more spiritual ones know neither limits nor obstacles. We are perfectly aware that many learned, as well as the unlearned, will object to such a novel theory of the distribution of the life-principle. They would prefer remaining in blissful ignorance and go on confessing that no one knows or can pretend to tell whence and whither this mysterious agent appears and disappears, than to give one moment's attention to what they consider old and exploded theories. Some might object on the ground taken by theology, that dumb brutes have no immortal souls, and hence, can have no astral spirits; for theologians as well as laymen labor under the erroneous impression that soul and spirit are one and the same thing.
Paralyzing the body frees the soul
But if we study Plato and other philosophers of old, we may readily perceive that while the "irrational soul," by which Plato meant our astral body, or the more ethereal representation of ourselves, can have at best only a more or less prolonged continuity of existence beyond the grave; the divine spirit — wrongly termed soul, by the Church — is immortal by its very essence. (Any Hebrew scholar will readily appreciate the distinction who comprehends the difference between the two words ruah and nephesh.) If the life-principle is something apart from the astral spirit and in no way connected with it, why is it that the intensity of the clairvoyant powers depends so much on the bodily prostration of the subject? The deeper the trance, the less signs of life the body shows, the clearer become the spiritual perceptions, and the more powerful are the soul's visions. The soul, disburdened of the bodily senses, shows activity of power in a far greater degree of intensity than it can in a strong, healthy body. Brierre de Boismont gives repeated instances of this fact. The organs of sight, smell, taste, touch, and hearing are proved to become far acuter in a mesmerized subject deprived of the possibility of exercising them bodily, than while he uses them in his normal state.
The attractive power of our thoughts
Such facts alone, once proved, ought to stand as invincible demonstrations of the continuity of individual life, at least for a certain period after the body has been left by us, either by reason of its being worn out or by accident. But though during its brief sojourn on earth our soul may be assimilated to a light hidden under a bushel, it still shines more or less bright and attracts to itself the influences of kindred spirits; and when a thought of good or evil import is begotten in our brain, it draws to it impulses of like nature as irresistibly as the magnet attracts iron filings. This attraction is also proportionate to the intensity with which the thought-impulse makes itself felt in the ether; and so it will be understood how one man may impress himself upon his own epoch so forcibly, that the influence may be carried — through the ever-interchanging currents of energy between the two worlds, the visible and the invisible — from one succeeding age to another, until it affects a large portion of mankind (Blavatsky, Isis Unveiled I, 178-181).