"If it doesn't sell, it isn't creative."
David Ogilvy

“Advertising says to people, ‘Here's what we've got. Here's what it will do for you. Here's how to get it.’”
Leo Burnett

“Your object in all advertising is to buy new customers at a price which pays a profit.”
Claude Hopkins

Advertising is, actually, a simple phenomenon in terms of economics. It is merely a substitute for a personal sales force - an extension, if you will, of the merchant who cries aloud his wares.”
Rosser Reeves

Source: The University of Texas at Austin, Department of Advertising http://advertising.utexas.edu/research/quotes/