Validity of Process

Modern science accepts that:

• a valid process is one that produces the results it says it is producing.
• valid results are the outcomes of a valid process.
• when repeated, a valid process always, reliably, produces the same results.

In Leeuwenhoek's time, these were revolutionary ideas. Where was the weight of authority? Aristotle? The Bible? These ideas about validity came from Francis Bacon and Rene Descartes, both of whom laid out the basic ground rules by which science still operates.

The founders of the Royal Society in 1660 were putting into action the ideas in Bacon's Novum Organum (1620) and New Atlantis (1627). Bacon said that the philosopher, what we now call the scientist, should abandon Aristotle's method. Instead, to discover cause and effect in nature, the scientist should proceed through inductive reasoning from fact of nature to law of nature.

Something observed by one person was more likely to be really there if another person at another time and place could observe it, too.

In Discourse on Method (1637), Descartes presented the four precepts that characterize his method. The precepts are on the left, and Leeuwenhoek's adherence to them is discussed on the No Longer Any Doubt page. They were very influential among the "natural philosophers" who were gathering regularly in London and Paris, outside the universities, to explore the implications of these new methods.

The Royal Society's Henry Oldenburg was the founding editor of its journal, Philosophical Transactions, that made a point of publishing observers and experimenters who followed these new, radical philosophies. Right away, he recognized that Leeuwenhoek was among them.

Philosophical Transactions's thousands of pages are filled with descriptions of things from all over the world by people from all over the world. In the sky, they tracked and measured everything they could see with a telescope. Here on earth, they counted chimneys and they opened cadavers. With a microscope, they looked closely at fleas and leaves, things they could already see.

According to Descartes' first precept, it was very important that the things they discovered were true. These moons and planets, cells and animalcules, had to really be there. They had to be doing what their discoverers claimed they were doing. In other words, they had to be valid.

When Nehemiah Grew and Robert Hooke had trouble repeating Leeuwenhoek's results, that is, had trouble demonstrating their validity, van Leeuwenhoek's response was to resort to experts. Birch's History notes that during the Royal Society's meeting of November 1, 1677:

Mr. LEEWENHOECK'S papers ... were read; four of which were testimonials of two ministers, a public notary, and other persons of good credit to the number of eight, of the truth of his former assertions concerning the almost incredible number of small animals wriggling in pepper-water; some of whom estimated, that they saw ten thousand, others thirty thousand, others forty-five thousand little animals in a single drop of water as big as a millet-seed.

However, that was not enough. The days of establishing validity by the witness of credible learned men were over. Leeuwenhoek was the first to admit that his claims about so many tiny animals were unbelievable. His observations would not be considered valid because ministers and notaries testified to their truth. The results had to be reliable, replicable, in order to be valid.

Early in his career, Leeuwenhoek tried to replicate the findings of Jan Swammerdam and Christiaan Huygens. Later, his showcase experiment for visitors was a validation of Harvey's theory about circulation of the blood. Toward the end of his career, he tested the ideas of Herman Boerhaave, professor in nearby Leiden.

In England, one of the main purposes of the Royal Society's meetings was to validate other scientists' results. The members would watch their curator of experiments, Robert Hooke, sometimes with help, perform experiments. Birch's History of the Royal Society is almost solely the notes and journals of these weekly meetings. Hooke's recently recovered Folio fills in a few gaps. The story of how the members went about replicating Leeuwenhoek's results is scattered over many meetings, the notes of which are often brief or incomplete.

The story on the related page Hooke's Three Tries -- about how the Royal Society came to validate van Leeuwenhoek's observations -- shows the new ideas at work.