There was a moment in the thirteenth century, when the great polymath Albertus Magnus held in his hand an exquisitely wrought ancient onyx cameo. He went on to describe it in beautiful and elaborate detail in his book On Minerals. His description is careful and exact enough that modern historians have been able to identify the specific cameo as one now in the Kunsthistorisches Museum in Vienna.Take a moment now to appreciate its details—it really is a gorgeous piece of work. Because of its relative hardness (a seven on the Mohs scale), its near-translucency, and its dramatic variations in colour, onyx makes for some of the most beautiful relief work that comes to us from Greco-Roman antiquity, and examples of such work can be breathtaking, as many a museum-goer can attest.So what is this object that we, now in communion with Albertus, have before us? A ten-centimetre-or-so pendant, bearing the silhouettes of two people in elaborately decorated military headgear. A snake adorns one helmet, as do a winged thunderbolt, a bearded man with horns, and a number of other decorative elements.But here is the rub: without even thinking about it, you and I immediately recognize the skill of the person who carved the cameo, who carefully removed layer after layer to take masterful advantage of the stone’s difficult-to-predict layers of colour. But—almost shockingly to my mind—Albertus did not see this skill at all. Not one whit of it. Instead Albertus thought the stone was produced by natural processes, in the earth, just like any other stones. This gorgeous, detailed cameo just happened. That, it seems to me, bears explaining.

Creatures Born of Mud and Slime is an attempt to make sense not just of how Albertus could think the way he did about this one object, but also to delve into the details of the mechanics, of the physical and biological forces, that were at play for him in its production. For Albertus this involved the stars and planets bearing down, in astrological influence, on the forces that shaped the production of stones here on earth. These forces were identical to, he tells us, the forces that produced the many kinds of animals that, like the cameo, were generated spontaneously. Like many before and after him, he theorized the process of spontaneous generation in remarkably subtle and well-informed ways. This raises important questions about what premodern approaches to things like this cameo can tell us about our own approaches to the natural world and how we understand observation, evidence, and theory today.Spontaneous generation as a mechanism for producing animals is generally seen today as a kind of simple, historical, ‘common-sense’ idea that grew out of people’s naïve inability to see, or even imagine, tiny things like fly-eggs landing on rotting meat. And so the idea that maggots just ‘come to be,’ directly, from rotting refuse is seen as a natural response by premodern thinkers arising from their impoverished imaginations. Creatures hits that idea with a hammer: a close examination of the sources who discuss spontaneous generation shows that people continuously questioned the idea of spontaneous generation in complex and sophisticated ways. Not only did they think hard about how such a mechanism could work for the generation of life from nonliving matter (it is very much a non-trivial problem), but they also continuously examined whether particular animals really were spontaneously generated or whether they could be shown to generate sexually instead.

And the evidence for spontaneous generation was, as I have said, really good, even (and especially) when it was examined very hard. The fact that spontaneous generation is the product of some really good investigating, while at the same time being no longer true, raises some important and difficult questions about science and its epistemology today.And when I say that spontaneous generation is no longer true, that is not quite right. Something like it did, after all, happen at least once. This positively infested earth that we live on is sufficient testimony to that fact.Some years ago now, I came across a casual reference in Ptolemy to a fact that he just took for granted about the world: that if you get garlic on a magnet it will wreck the magnet. Tracing the history of this claim­­­—and it gets reiterated no small number of times—to its eventual dissolution proved a fascinating exploration of how obviousness infects our thinking about the world (see my What Did the Romans Know? for the details). When I started on the project the would become Creatures Born of Mud and Slime, I thought that spontaneous generation might work in much the same way as garlic-magnets. I was wrong.Instead, spontaneous generation is a much richer, much more complex, and much more wonder-filled ground in which to explore fundamental questions about how people think about evidence, plausibility, matter and material forces, and processes that somehow transcend the materiality of those forces. True, as with garlic and magnets, what we think of as obvious is challenged again and again as we engage with very different ways of looking at the world and different ways of making sense of our experiences in it. That, to my mind, is both humbling and fascinating. I suppose it's why I'm in this business.