I've thoug- pondered about this 8]

There are circumstances

First, I recognize the image in the background, from the Rider-Waite Tarot deck :D -- second, in principle, the past (according to the present) truly is but a dream... that's sort of depressing in a way, but I like to think of it like a story in a book. You must learn from it, and apply it to the present and future. The same as you should ACTUAL dreams. For - as of yet - humans have not found a concrete way of returning to the past... but there would BE no future or present if there were no past. Certainly, humans have the capacity to erase our entire existence (including the past/future) - yet we haven't as of today... although that - in itself - is due to the fact that we have learned from the dreamy like occurrences of the past. This (the present) may well be the same as the past - as undeniably - this will BE the past in a very short amount of time. But who knows - there have been important philosophers have entertained the thought that this existence, our consciousness - may be nothing but a dream itself. But personally, I don't think that's for us to know - at least not while we reside in these meat sacks on this rock hurtling through space. That answer is unveiled at a later point in 'time.'