The Limitations of Evidence

Karl Popper's most distinctive contribution to the episte­ mology of science was his contention (itself an hypothesis) that sound scientific knowledge is based on the develop­ ment of testable hypotheses, that may then be falsified by the new evidence that emerges from the process of testing. His Logik der Forschung (1934) has been both translated1 and elucidated for the general reader2 by Popper himself, and vested with elegance and plausibility by Peter Medawar in Pluto's Republic*. Imaginative hypothesis followed by meticulous testing for concordance with what is already known, or ascertainable by trial (Popper's 'hypotheticodeductive process') is probably closer to the reality of scien­ tific discovery than the alternative model of Baconian induction from raw facts. It is also appealing, in asserting the importance of imagination in science, in contrast to the 'dry-as-dust' stereotype that is all too prevalent among those whose acquaintance with actual scientists is limited. The story of science blazes with what Michael Polanyi called 'passion and controversy"1. Moreover, Popper's model emphasises the provisional or contingent nature of scientif­ ic knowledge even a Newton awaits his Einstein, and any beautiful theory is vulnerable to an ugly fact. A scientist who embraces hubris is living beyond his intellectual means. Nevertheless, total surrender to the scepticism that is theoretically demanded of the scientist is in itself unwise; there are degrees of probability so high that pragmatically, though not in absolute theory, they carry virtual certainty as Henry Harris puts it: 'I do not believe that it will ever be shown that the blood of animals does not circulate; that anthrax is not caused by a bacterium; that proteins are not chains of amino-acids'5. Of course, these examples are matters of direct observation (once you know how), and not to be confused with the broad paradigms that concern the philosopher of science, and which are subject to radical changes, or 'revolutions'6 at (fortunately) long intervals. Active scientists are in general indifferent to the origin and even the validity of the conceptual framework that they accept and within which they work as Medawar pithily puts it: 'the history of science bores most scientists stiff. A great many highly creative scientists... take it quite for granted, though they are usually too polite or too ashamed to say so, that an interest in the history of science is a sign of failing or of unawakened powers'3. Happily, a flawed framework does not prevent all accretion of knowledge through observation. The Greeks could add to physical science within a framework tom between Leucippus' atoms in a void and the four elements of Empedocles; somewhat