Those who believe in a pre-mortal, un-incarnated, spirit existence need to have an answer to the question of why so many people apparently cannot remember anything about it.
The usual example is that there is a 'veil' placed between that phase of our lives, and now; between the eternal lives of Men and Angels dwelling in Heaven, and life on earth; and this veil is necessary to the fulfilment of our mortal tasks. We need to be on our own.
The difficulty is that while this veil explanation works for many phenomena, it doesn't account for the important fact that communications do apparently 'pierce' the veil, from time to time - sometimes those in Heaven seem to communicate with those on earth, and seem aware of the activities of those on earth.
Also, how to account for those of us who do feel a strong (albeit extremely imprecise and incomplete) conviction and memory of the reality of pre-mortal existence - and the numerous reports of such an experience throughout human history?
For example, evidence of pre-mortal life is widespread if is seen to be present in the form of belief in re-incarnation.
I interpret the intuitive belief in systematic reincarnation in such terms. I think it probable that true reincarnation is very rare indeed - done only for special divine purposes; but that the intuitive belief so many people have about their own personal reincarnation, is in reality a rational misinterpretation of what are actually true memories of their own pre-mortal life.
Anyway; why set up a veil that is partial and incomplete, and sometimes intended to be breached - when the veil amounts to a total obscuration for some people but not for others?
Well, such objections to 'the veil' are not critical. All mortal understanding is metaphorical (even when true), and all metaphors are incomplete and break-down when pushed. The metaphor of the veil may serve for the most important purposes.
However, my own understanding of the apparent-veil is different; my understanding is that there is not so much a material barrier, but a barrier of thought-forms which divides pre-mortal and mortal life. Specifically that, when looked at from a Heavenly perspective, mortal life is extremely slowed-up, as if we lived in a more viscous medium than Heaven.
The spirit existence of pre-mortal life was swift and immediate - there was no significant gap between thought and action because there was no body, and because the Heavenly environment had little resistance.
Heavenly life, and thought, was fluid and frictionless.
Earthly life, by contrast - and necessarily, as being vital to the purpose of it - is slowed-up and delayed.
Mortal, earthly life is experienced as having resistance; there is resistance interposing between the spirit and the body, and events (both creation and corruption) unfold in slow-motion (compared with Heaven) - sometimes with what is experienced as painful slowness.
Patience and prudence are always necessities in mortal life; and courage of course - exactly because of the potential for suffering, and the gap between the ideal and the actual.
From this I infer that our mortal memories of pre-mortal life are always present and for everyone - if we choose to introspect - but that these memories are of a life that was so swift and fluid and frictionless, that from our earthly perspective they are a blur.
Our memories of pre-mortal life are (by analogy) somewhat like watching a video recording sped-up a thousand-fold: we see just a blur of shapes and colours and sounds, creating a general impression that is mostly un-interpretable, but from which we may occasionally perceive the flicker of a recognisable picture or soundscape.
For those who are most attuned to these pre-mortal memories, most gifted and skillful at interpreting them, the experience may be perceived as an 'instantaneous' understanding of so many simultaneous things as to be indescribable.
Furthermore it is extremely difficult for our slow, viscous, meaning-oriented mortal memories to retain this kind of ultra-sped-up information - there is just far too much stuff to take-in and store, its sequence cannot be properly perceived, the elements cannot clearly be resolved.
I would push this metaphor even further. Quite often, reports of knowledge of other worlds, other lives, of Heaven has been experienced in an opposite way to that I have described: experienced as a static state-of-being - such as Nirvana. Experienced as if it was mortal life on earth that was 'swift and slippery', and Heaven that was unchanging: an eternal, unitary state of being.
My explanation is that when something is sped-up fast-enough, it becomes perceived (by mortal minds) as static and unchanging.
Try the experiment of doing this with music - as the playback is sped-up, at first the music is experienced as faster and faster, but a line is crossed when the notes blend together, and a much slower and more gradually modulating chord-like sound emerges (representing the overall dynamics, the average pitch, the tonality etc.).
This is my explanation for how Heaven is perceived by mortal minds, and how the limitation of earthly human perception may mistake what are in reality extreme degrees of swiftness and fluidity for (what appears to be) slow, gradually modulating, even static states of being.