Preference revelation

[Ananda Gupta]

People say they want things.  But when it comes time to bear the costs of 
having those things, they change their minds.  That is, people's true 
preferences can be best discovered through observing their actions, not 
their words.

Does anyone know of any formal economics literature having to do with the 
above proposition?  Are there any articles considered classics (since that 
does seem to be a basic issue in economics)?

-ASG