The principles of science — Context and Discussion

The Physicist's Unspoken Method

Jevons begins by asserting that scientific method is often used without being explicitly described. He points out that physicists are usually too engrossed in accumulating details of their special sciences to reflect on their own reasoning. This observation is not merely prefatory; it frames the entire work as an attempt to make explicit what practitioners do implicitly. Jevons argues that the study of method is especially needed for those who wish to introduce scientific order into less methodical branches of knowledge. The implication is clear: logic is not an abstract discipline but a practical tool that can improve inquiry across all fields.

Fallacies of Observation and Non-Observation

Jevons devotes considerable attention to errors arising from faulty observation. He classifies cases where perception of one event leads to mistaken inference about another. For example, he discusses the fallacy that the moon influences clouds, noting that the full moon is most visible on cloudless nights, creating a spurious correlation. He cites Mr. W. Ellis, who showed from Greenwich Observatory data that the moon has no appreciable power over clouds. Jevons also examines negative arguments from non-observation, warning that the absence of evidence is not evidence of absence. He gives examples from geology, where the supposed absence of human remains with extinct animals was later disproved by flint implements.

The Role of Unseen Phenomena

Jevons emphasizes that many natural processes are invisible to us. He describes how upward and downward air currents are only revealed by cumulous clouds, and how higher atmospheric motions become apparent only through cirrus clouds or auroral displays. He extends this to comets, suggesting that their visible tails arise from previously invisible material that becomes condensed or electrified. These examples illustrate a key methodological principle: we must interpret our experience in an affirmative sense only, and avoid concluding that what we do not observe does not occur. Jevons's careful attention to the limits of perception shows his commitment to grounding scientific method in the realities of human observation.

Inductive Inference and Its Pitfalls

Throughout the excerpts, Jevons demonstrates a keen awareness of how inductive inferences can go wrong. He discusses the fallacy of mistaking correlation for causation, as in the moon-cloud example, and the danger of relying on negative evidence. He also notes that our attention may be drawn to coincidences that are actually unrelated. Jevons's approach is to dissect these errors with precision, often citing specific researchers and data. His method is not merely critical; he aims to provide a constructive framework for valid reasoning. The excerpts show a writer who is as concerned with the psychology of observation as with formal logic, blending empirical examples with logical analysis.

Jevons's work is best approached as a detailed examination of how scientists actually reason, rather than as a prescriptive manual. Readers will benefit from attending to his concrete examples—the moon fallacy, the non-observation of human fossils—which ground abstract principles in real scientific controversies. The book rewards careful reading of its case studies, where Jevons's analytical method is most vividly displayed.