Louis Pasteur

The name attached to germ theory and vaccination belonged to a working chemist and microbiologist, not a theorist at a distance from the bench. That pairing — germ theory and vaccination — is the shorthand by which his influence is still summarised, and it stands as much for a way of arguing as for a set of results.

He was born in Dole, France, in 1822. His training combined strict attention to method with a willingness to synthesise across problems, and his early work is characterised less by startling novelty than by the clarity with which it set out what was actually at stake. Inherited assumptions were examined rather than accepted; vague problems were reformulated into ones that could be tested. Correspondence, travel and steady reading built up a working repertoire, and apprenticeships and collaborations gave him a craft ethos that held.

Mid-career brought consolidation and a wider range of problems. Rivals and critics played a real part in this: the pressure of opposition sharpened both his arguments and the way he presented them, and a series of publications, lectures and demonstrations turned tentative positions into settled statements. He moved between experiment and theory rather than committing to one, and failed attempts functioned as tests that improved the next design. Students and correspondents carried the approach beyond any single institution.

Later years went to systematising the work — notes, editions and reflective writing intended to fix the best version of the central insights. The surviving drafts and corrections show both the rigour and the contingency of the enterprise. He died in 1895. Institutions have commemorated the achievements since, but the more durable inheritance is procedural: the work is still reperformed in laboratories and classrooms, which makes the legacy a method rather than a monument.