Unlike the
future, the recent
past is literally and figuratively an open
book to us - we see it with the clarity of having seen its
consequences as well, and 20/20 is simply another way of expressing that clarity. It is a measure of what is considered
normal eyesight. If you have 20/40
vision, you have to stand 20 feet from a
sign to read it, while a normal person could stand at 40 feet and read it.
In some writings of
fantasy, there are beings that see the
future with clarity, but have no
memories of the past - once an event has happened, they have no
knowledge of it (see
foregoblin). Foresight is 20/20.