i always thought that it was Menger's Principles (1871).  The IHS 
edition 1976 pg 127 clearly shows diminishing 'satisfaction' in 
Menger's table, as well as the text surrounding the table describes 
this decreasing marginal utility (although menger calls it 
satisfaction not utility) relative to consumption of additional units 
of one good compared to other goods.

Cameron Weber









 > Date: Sun, 8 Nov 2009 11:21:34 -0500
 > From: [log in to unmask]
 > Subject: SHOE: QUERY--Diminishing marginal utility
 > To: [log in to unmask]
 >
 > Dear colleagues,
 >
 > I'm looking for the first occurrences in the history of economic thought of
 > the idea of diminishing marginal utility. Would you have any suggestions?
 >
 > Many thanks!
 >
 > Axel Gosseries


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