Visited by John Locke

May 1, 1686

Under suspicion of plotting to asssassinate King Charles II, John Locke fled England in 1683. The portrait below (click to enlarge) by John Greenhill was painted ten years earlier. Locke lived in Holland -- Utrecht, Amsterdam, and Rotterdam -- until he accompanied the future Queen Mary back to England in February 1689.

While he was in Holland, Locke became acquainted with the leading Dutch intellectuals and wrote some of his major works, including Essay Concerning Human Understanding. He also kept a journal. The entry for June 22, 1686, tells of his visit to Leeuwenhoek's house and laboratory in Delft.

Leeuwenhoek showed Locke red blood cells, a human tooth, and dog sperm. Locke regretted that he had not seen the best lenses:

The glasses we saw in, he said, would magnify to a million, which I understood of cubicle augmentation, which is but 100 in length. ... The best of all his glasses ... we saw not nor (as I heare) does he show them to others.

After his return to England, Locke published Essay Concerning Human Understanding. It emphasizes the importance of knowledge coming from direct sensory perception, so looking through Leeuwenhoek's lenses must have fascinated Locke. The Essay contains a passage that does not mention Leeuwenhoek by name, but seems to have been rooted in his visit:

For how much would that man exceed all others in knowledge, who had but the faculty so to alter the structure of his eyes, that one sense, as to make it capable of all the several degrees of vision which the assistance of glasses (casually at first lighted on) has taught us to conceive?

What wonders would he discover, who could so fit his eyes to all sorts of objects, as to see, when he pleased, the figure and motion of the minute particles in the blood, and other juices of animals, as distinctly as he does, at other times, the shape and motion of the animals themselves?

But to us, in our present state, unalterable organs so contrived, as to discover the figure and motion of the minute parts of bodies, whereon depend those sensible qualities we now observe in them, would perhaps be of no advantage. God has, no doubt, made them so, as is best for us in our present condition.