Here's a nice example illustrating the difference between a valid argument with false premises and a sound argument (which must be both valid and have all true premises). It comes from a "profile" of Nobel-prize winning economist Paul Krugman: MacFarquhar, Larissa (2010), "The Deflationist: How Paul Krugman Found Politics", New Yorker (1 March): 38-49. http://www.newyorker.com/reporting/2010/03/01/100301fa_fact_macfarquhar Here's the quote, with my comments interpolated {in braces}: "[F]ailure to represent reality accurately is rarely a fatal flaw in an economics model--what's valued is the model's usefulness as an analytic tool. {I.e., failure to have true premises is rarely a flaw. What's valued is a model's validity.} "The most successful paper Krugman ever wrote was about target ones, and it was completely wrong. {That's a bit too strong. It turns out that it was valid, so it wasn't "completely" wrong; but it had a false premise, so it was, in some sense, "wrong".} "In the years before Europe adopted the euro, it was thought that establishing something between floating exchange rates and fixed ones--a 'target zone' within which a currency would be allowed to float--might reap some of the advantages of each. He [Krugman] estimates that by the time the paper was officially published, in 1991, some hundred and fifty derivative papers had already appeared. {I.e., other economists drew further (presumably valid) inferences from Krugman's conclusion.} "'Empirically, it doesn't work at all,' Krugman says. 'People loved it as an academic thing, but it had some very strong predictions about interest rates inside target zones. Those predictions all turned out to be wrong. But nobody attacked me for that. I was showing that if target zones worked the way that people say they're supposed to work, then this is how it would play out." (p. 45) http://www.newyorker.com/reporting/2010/03/01/100301fa_fact_macfarquhar?currentPage=7 {I.e., "empirically", his premise was false. "Academically", it was a valid argument. The predictions were wrong, because they were based on a false premise, not because they didn't follow validly from that premise. As Krugman says, he showed "that IF target zones worked" that way, "THEN this is how it would play out". I.e., he showed the validity of that argument.}