p6.
Karl Popper (1945), The Open Society and Its Enemies. Karl Popper’s most succinct statement of falsifiability was in his first publication, a two page letter in the periodical Erkenntis in 1933: “Scientific theories, if they are to deserve the name ‘scientific’, should be at least semi-decidable, one-sidedly decideable.” That is, they might not be proved, but they might be disproved. Cited in Imre Lakatos (1973), Lectures on Scientific Method, lecture seven, “Falsification and Intellectual Honesty”, p 89. For a valuable summary of falsifiability, see Brian Magee (1973), Popper, chapter 3.