Wrote Letter L-190 of 1687-08-06 to members of the Royal Society about the calander and the louse and against spontaneous generation

August 6, 1687
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Text of the letter in the original Dutch and in English translation from Alle de Brieven / The Collected Letters at the DBNL - De Digitale Bibliotheek voor de Nederlandse Letteren.

The manuscript is lost. The transcript in Alle de Brieven / Collected Letters is from Vervolg der Brieven.

There is no record in Birch's History that this letter was received or read at a weekly meeting.

The first third of the letter is also published in Birch, p. 652. It includes a greeting and first paragraph not published in Vervolg der Brieven or, consequently, Collected Letters. In Birch, the letter is addressed to “the noble lords, gentlemen, and members of the Royal Society”. It begins, “My last was of the second of July, which no doubt was duly delivered; at present, I send further some of my small observations.” Letter L-189 was in fact dated 11 July,

Souls and miracles

Based on the breadth of his observations and his principle of a great degree of uniformity in nature, Leeuwenhoek generalized from these beetles to all living creatures:

I definitely assert that, just as I have now clearly proved with regard to the calanders; that they cannot originate except through propagation, so it must also be with all creatures that are endowed with movement; (that which we call, in animals, a living soul) for however small it may be, its first production depends upon the beginning of Creation.

If this were otherwise; namely, if from immobile substances such as stone, wood, earth, plants, seeds, etc. a body came forth that (as said above) was mobile, that would be a miracle and its production would once again be dependent upon the great almighty Creator.

It is unclear whether the calander beetle's soul had the same nature and status as the human being's soul. Still, Leeuwenhoek had the problem of where this soul came from. Did each one of the "unimaginable large number" of little animals in male seed have a soul? He called them living (levende), so apparently they did. Not only that, all of these souls had existed since the beginning of creation.

And just as I said in one of my foregoing missives, that no tree is newly-made, but that they all depend upon the beginnings, or young plants that are in the seeds; so all creatures that are endowed with a moving or living soul, depend upon their first generation; or to put it in a better way, they depend upon the moving or living animals that were made in the male seeds, in the beginning of Creation.

This will probably seem strange to some people, and they might perhaps refer me to the propagation of other small animals; but whoever wants so much work as I have spent on this examination of the calander (which has occupied me, on and off, for more than four months).

Leeuwenhoek was struggling to reconcile what he saw before his eyes and the assumptions of his time about the nature of living things that stretched back at least to Aristotle.=