Full Idea
The economic principle of marginal utility states that for a given individual a set amount of something is more useful when the individual has little of it than when he has a lot.
Gist of Idea
'Marginal utility' says something is more useful if it is in short supply
Source
Peter Singer (Practical Ethics [1979], 02)
Book Reference
Singer,Peter: 'Practical Ethics' [CUP 1989], p.22
A Reaction
But individuals may very a lot on this one. 'He can't get enough of X'. I may be desperate to buy 10,000 books, but you may consider such a need ridiculous, so who decides?