Philosophy of Natural Science

Summary

Philosophy of Natural Science is a short introductory textbook by the German-American philosopher Carl G. Hempel, published in 1966 by Prentice-Hall in its Foundations of Philosophy series. It is still under copyright, so the library holds no text; this record describes the book in its own words and quotes only two short sentences. In about a hundred pages and eight chapters Hempel sets out how hypotheses are formed and tested, what it means for a law to explain an event, and how theories and theoretical concepts relate to observation, using concrete episodes from the history of science rather than formal logic. The first chapter distinguishes the empirical sciences, which must be tested against experience, from the non-empirical disciplines of logic and pure mathematics. The second chapter, the most widely reprinted, tells the story of Ignaz Semmelweis at the Vienna General Hospital in the 1840s. Hempel follows Semmelweis as he considers and rejects explanations for the high death rate from childbed fever in the First Maternity Division: epidemic influences, overcrowding, rough examination by students, the fear caused by a priest's bell, and the women's position in delivery. The death of his colleague Kolletschka from a scalpel wound during an autopsy suggested that students coming from the dissecting room carried 'cadaveric matter', and washing in chlorinated lime brought the death rate down sharply. From the story Hempel draws the moral that hypotheses are not mechanically derived from collected facts but are guesses invented to account for them, to be judged by testing their consequences. The following chapters analyse the logic of testing. A failed prediction can refute a hypothesis, but a successful one cannot prove it; tests always rely on auxiliary assumptions; crucial experiments and ad hoc rescues are examined, with examples from Torricelli and Pascal on the weight of air and from the phlogiston theory. Hempel discusses what makes evidence strong: quantity, variety, precise new predictions, theoretical support and simplicity. Chapter 5 presents his best-known idea, the covering-law model, in which an event is explained by deducing it from general laws and particular conditions, with a statistical variant for probabilistic laws. The final chapters treat theories that posit unobservable entities, operational definition of scientific concepts, and whether biology can be reduced to physics and chemistry. Widely used in college courses for decades, the book was translated into many languages and remains one of the clearest short statements of the logical empiricist view of science.

Top Lessons

• Facts do not arrange themselves

  Hempel argues that no one could make progress by first collecting all the facts and only then looking for patterns, because without a hypothesis one does not know which facts are relevant. Semmelweis needed a guess about what killed the women before he knew what to observe. Imagination begins inquiry; testing disciplines it.

• A test can refute more than it can prove

  If a hypothesis predicts something that does not happen, it is in trouble; if the prediction succeeds, the hypothesis is supported but not proved, because some other hypothesis might have predicted the same thing. Hempel teaches students to see the asymmetry and to ask how much support a successful test really gives.

• To explain is to show why it had to happen

  On Hempel's covering-law model, explaining an event means showing that it follows from general laws together with particular conditions. The model makes explanation and prediction two sides of the same reasoning. Later philosophers questioned whether every explanation fits this form, and Hempel himself refined it.

• The cost of an idea before its time

  Semmelweis reasoned well and saved many lives, yet his conclusion was resisted for years. Hempel's chapter shows the method that led to the right answer; the history around it shows that method alone does not guarantee acceptance.

Verified Quotes

Medical Lens

The book is philosophy of science, but its most famous chapter is a case study in clinical epidemiology: Semmelweis's comparison of mortality from puerperal fever in two maternity divisions of the Vienna General Hospital (1844–1848), his elimination of rival explanations, the inference from Kolletschka's death after an autopsy wound, and the trial of handwashing with chlorinated lime. Hempel reports the yearly death rates and the later extension of the hypothesis to putrid matter from living patients after a case of uterine cancer. The chapter is often taught to medical students as an introduction to hypothesis testing and confounding, and it stands just before the germ-theory papers of Pasteur and Lister in the history of infection control. Hempel's discussion of reduction in the last chapter also touches the old debate between mechanism and vitalism in physiology.

• Ch. 2

  Semmelweis rejects epidemic influences, overcrowding, rough examination, the priest's bell and delivery position as causes of the excess mortality in the First Division.

• Ch. 2

  Kolletschka's fatal scalpel wound during an autopsy suggests that students coming from dissections carry cadaveric matter; handwashing in chlorinated lime follows.

• Ch. 8

  Mechanism and vitalism in biology and the question whether life processes can be reduced to physics and chemistry.