Out of sight, out of mind
The idea that something is easily forgotten or dismissed as unimportant if it is not in our direct view.
The use of 'in mind' for 'remembered' and 'out of mind' for 'forgotten' date back to the at least the 13th century. The earliest printed citation of a link with memory and the sight of something is in John Heywood's Woorkes. A dialogue conteynyng prouerbes and epigrammes, 1562, as reprinted by the Spenser Society, 1867:
"Out of sight out of minde."
The phrase is used as an example of the supposed comic results that early computer translation and speech recognition programmes came up with. The phrase 'out of sight, out of mind' was supposed to have been translated by a computer as 'invisible idiot', 'blind and insane' etc. This is on a par with 'computers can wreck a nice peach' (computers can recognise speech), which is also used as an example of how computers lack the general knowledge to compare with humans at speech recognition.
These reports lack consistency and are too neat to be anything other than inventions. There's no evidence to support the stories but they do illustrate that although 'anyone can make a mistake, but to really foul things up you need a computer'. Even using recent (2007) programs to translate 'out of sight, out of mind' into Russian and then back to English the best they could do was 'from the sighting, from the reason'.
See also: the List of Proverbs.