Murray Rothbard on economic methodology

Human life is not a laboratory, where all variables can be kept fixed by the experimenter, who can then vary one in order to determine its effects.  In human life, all factors, including human action, are variable, and nothing remains constant.  But the theorist can analyse cause-and-effect relations by substituting mental abstractions for laboratory experiments.  He can hold variables fixed mentally (the method of assuming 'all other things equal') and then reason out the effects of allowing one variable to change.  By starting with simple 'models' and introducing successive complications as the simpler ones are analysed , the economist can at last discover the nature and operations of the market economy in the real world.  Thus the economist can validly conclude from his analysis that 'All other things equal (ceteris paribus), an increase in demand will raise price.'

~ Murray Rothbard, An Austrian Perspective on the History of Economic Thought, Vol. I, Chapter 12, "The founding father of modern economics: Richard Cantillon," p. 348