Many people have an intuition of pre-mortal existence, a very vague memory or feeling or hunch that their life did not start at birth (or conception).
They may not take notice of it, or it may be explained by reason or revelation. So, some attribute this to reincarnation, others to a spirit life before incarnate life... there are various possibilities pagan, philosophical, mainstream Christian and nowadays Mormon - documented by Terryl Givens in When Souls had Wings.
But a big problem is trying to explain (usually by some metaphor) why we don't remember more of this pre-mortal existence - or more exactly why we remember enough to have an intuition, but not enough to recall anything specific.
The main metaphor I have come across is that a 'veil' divides the pre-mortal from the mortal life: the idea apparently being that through a veil one can perceive shapes and movements, but no specific details.
On the other hand, the metaphor breaks down in the sense that a veil is so flimsy, yet the barrier preventing us from perceiving pre-mortal existence seems almost absolute; also that a veil seems suited to dividing an 'ongoing' spirit-world from this mortal world, but less helpful in explaining the vague nature of our memories.
Another way of thinking about this, another metaphor, is that mortal life could be conceptualized as extremely thick and viscous; so that compared with pre-mortal life we can only wade and move very slowly through it, and with great effort.
I get the general idea from William Arkle - the point being that the viscosity and extremely slow movement of earthly life, relative to Heavenly or other 'higher' forms of life, is an essential aspect of our unique ability to gain experience as incarnate mortals; BUT that it creates a significant (although not absolute) barrier to our perceptions and memories of pre-mortal existence - which seems to us almost incomprehensibility swift and frictionless.
Just a thought...