This is a bit of further detail on a topic I didn’t have space to treat at length in my TLS article on Diderot, but which always had particular interest for me. In the 18th century, philosophy of mind was struggling in several different ways to come to terms with the influences of empiricism and naturalism. Hobbes was arguably the first to really press the point for a monist, materialist view of reality, life, and the mind, but it wasn’t until the 18th century that in-depth accounts were constructed. I find Diderot’s to be the most satisfying, but it is also one of the most radical.

Among those, like most of the philosophes, who had embraced atheism or at least a non-active role for God in the process of the development of the universe and the human, there were two general schools of thought. One went by the names of vitalism and hylozoism, two separate but related concepts. Both propose that there is some innate principle or characteristic of some or all matter that gives it life, sensitivity, and/or mind.

(The difference between raw sensitivity and higher cognition/mind was another significant question, but which I’ll avoid here since it will only complicate matters. The question at hand is how to explain the occurrence of any sort of sensitivity or mental properties whatsoever in some matter.)

The second school proposed that sensitivity and mind was an emergent property, only arising out of matter when it had a certain configuration and/or relation to the world. This appealed to the more mechanistically-minded.

These two schools have strong analogues today, though bits and pieces have switched sides or been subdivided. Vitalism and hylozoism posit a certain innate property to some or all matter in the same way that John Searle’s biological naturalism or Galen Strawson’s panpsychism does (David Skrbina’s Panpsychism Through the Ages traces this line), while the emergent school continues today in certain forms of functionalism.

The question still remains, ultimately, one of whether sensitivity or mind is a fundamental or emergent property.

Forever ecumenical, Diderot seems to draw from both sides. Diderot proposes at the start of D’Alembert’s Dream that a stone could potentially think, possessing a “latent sensitivity,” and extends this sort of property to all matter as a kind of “pansensism,” tantamount to panpsychism. On the other hand, he gives a lengthy account of how matter spontaneously organizes itself into increasingly complex forms so as to produce hierarchical structures of consciousness and experience. Here he seems to anticipate Daniel Dennett’s homuncular functionalism. And he flatly denies that there are prototypical forms of sensitive organisms that can combine to form larger ones.

Lester Crocker thinks Diderot never figured it out:

How matter organizes itself into living forms is a question that perplexes Diderot in all his writings on the subject. Does inorganic matter become life by restructuring itself to produce “sensitivity”? Or does “sensitivity” exist in latent or degraded form in non-living substances, awaiting activation by some process such as chemical fermentation (spontaneous generation) or ingestion? He is never able to decide between the two alternatives, or indeed to believe for long, with any real conviction, in either one.

Lester Crocker, Diderot’s Chaotic Order

I think Crocker is wrong here. Diderot had sufficient perspective to find neither approach wholly satisfactory, and so he attempted to combine the best aspects of each. The key is in a phrase that he used near-identically in both Elements of Physiology and D’Alembert’s Dream: “la sensibilité, propriété générale de la matière, ou produit de l’organisation.” [Sensitivity, general property of matter, or product of organization.]

This phrase may seem like dithering, but it is in fact Diderot specifying coexistent causes of sensitivity. Elsewhere he makes it clear that neither description by itself is sufficient; he is not giving a binary opposition.

This less than clear point is a result of Diderot’s dynamic metaphysics. Diderot was rather Heraclitian in his metaphysics: the nature of the universe was constant change, best represented by spontaneous biological evolution and organization. But he extended this to all matter, specifying properties as potentialities rather than actualities. Here, he physicalizes Leibniz. To quote another excellent Diderot scholar, Marx Wartofsky:

Diderot’s matter has motion as an inherent property. It is not endowed with motion; it is not a ground in which motion is put. Matter itself is uncreated, eternal, its motion is its essential mode of existence.

The “latent sensitivity” is not akin to potential energy in the conventional sense, as Diderot thinks of such potentialities as primary properties, not abstractions.

Mais quel rapport y a-t-il entre le mouvement et la sensibilité? Serait-ce par hasard que vous reconnaîtriez une sensibilité active et une sensibilité inerte, comme il y a une force vive et une force morte? Une force vive qui se manifeste par la translation, une force morte qui se manifeste par la pression ; une sensibilité active qui se caractérise par certaines actions remarquables dans l’animal et peut-être dans la plante; et une sensibilité inerte dont on serait assuré par le passage à l’état de sensibilité active.

D’Alembert’s Dream

So while a latent sensibility is rendered active depending on the particular state of matter at a given point, the property is always there. This is not just terminological juggling. Diderot’s suggestion is that reality is primarily processual and makes sense when modeled as such. With such a dynamism at the root of his metaphysics, he is able to posit a “latent” property that does not take the form of a measurable static quality but as a description of a process fundamental to all matter.

Diderot is not certain how latent matter becomes non-latent, and I think this accounts for some of his own hesitation and confusion in dealing with this topic. He gives examples of it happening, as with eating food, but he sees this as a scientific problem to be solved when greater resources are available. Diderot’s contribution was to reframe the problem of mind metaphysically in order to make the possible answers more satisfying.