This essay explores the epistemologies of art and science through the examples of Max Ernst (Dadaist and Surrealist artist) and Ernst Mach (physicist and forerunner of the Logical Positivists). To extrapolate from one example to an entire discipline may be problematic, but it turns out that both men demonstrate universals in their approach, oeuvre, and legacy. It is argued that the philosophy of science is less useful in understanding the epistemology of physics than a contemplation of the practice of the physicist. By contrasting it with the practice of the artist we can find both epistemological similarities and differences. However it is proposed here that an epistemology of art has to start with art practice, without the preconceptions of a ‘scientism’ that too readily skew such an investigation. The work of Ernst Mach shows that his account of the methods of physics is more tentative and fraught with questions of the subjective than the intellectual legacy he spawned, and hence of value in examining the epistemology of art.