Out of sight, out of mind.
What is not present is quickly forgotten.
What They Mean
Attention is a finite resource with no archive. Remove the stimulus and the attachment follows shortly after.
Are They Right?
More often than anyone likes to admit. The mind is tidy and ruthless about shelving what it cannot see.
What They Left Out
Certain things remain vigorously in mind regardless of sight, usually at three in the morning.
They Also Say
Read the ruling: Absence Makes the Heart Grow Fonder vs. Out of Sight, Out of Mind