Edward Feser, Aristotle’s Revenge: The Metaphysical Foundations of Physical and Biological Science (Neunkirchen-Seelscheid: Editiones Scholasticae, 2019), 332-334.

...an objection grounded in modern physical science needs to be addressed. For it might appear that science has vindicated the reductionist ambitions of atomism at least to a large extent even if not completely. For example, it might be claimed that modern chemistry has shown that water is really nothing but hydrogen and oxygen, that modern physics has shown that the facts about elements like hydrogen and oxygen are in turn reducible to quantum mechanics, and that a similar story can be told about every other object or stuff familiar from ordinary experience. Hence, even if hylemorphism gets the big picture right, it might appear that it is still largely wrong about the everyday world. It wins the war, but only after losing most of the battles.

[...]

Yet you don’t need to be an Aristotelian to reject this characterization of the situation. For work in contemporary philosophy of chemistry casts doubt both on the claim that chemistry affords us reductionist accounts of ordinary substances, and on the claim that physics affords us a reductionist account of chemistry. Indeed, in their Stanford Encyclopedia of Philosophy article surveying the field (2011), Michael Weisberg, Paul Needham, and Robin Hendry speak of an “anti-reductionist consensus in the philosophy of chemistry literature.”

One of the problems is usefully approached by way of John Locke’s account, in Book Ill of the Essay Concerning Human Understanding, of the relationship between nominal essence and real essence. The nominal essence of a substance like water is essentially the collection of attributes common sense groups together under the term “water,’’ such as being liquid at room temperature, being clear, boiling when heated, turning to ice when frozen, and so forth. The real essence is the corpuscular structure of water as revealed by physical science, on which the attributes associated with the nominal essence depend. The nominal essence might be said to correspond to the “manifest image” of water, and the real essence to the “scientific image” of water (to borrow Sellars’ language).

Now, I certainly wouldn’t endorse everything Locke has to say about essence. [...] But among the useful points he makes is that the relationship between nominal and real essences is reciprocal. On the one hand, deep and truly scientific knowledge of a thing cannot rest at the level of the nominal essence, but requires penetrating to the underlying real essence. That much might sound like grist for the reductionist’s mill. But at the same time, Locke tells us that a real essence is always the real essence of a species or class of things; and it is the nominal essence that determines what species or class a thing belongs to (Essay, Book III, Chapter VI, paragraphs 6-7). Hence, while only a grasp of the real essence of water gives us a deep and scientific knowledge of its nature, we couldn’t know in the first place that what we thereby grasp the nature of is water, specifically, unless we knew the nominal essence. Real essence, you might say, “piggybacks” on nominal essence, and the scientific image on the manifest image. Our grasp of the former presupposes a grasp of the latter.

J. van Brakel ([Philosophy of Chemistry (Leuven: Leuven University Press. 2000)], pp. 73-82) deploys this point in criticism of facile reductionist claims about water and H2O. To forestall misunderstanding, note that the question is not whether modern chemistry is correct to identify hydrogen and oxygen as the constituents of water. Of course it is correct; no one denies that. What is at issue is the idea that water is nothing but hydrogen and oxygen in a certain arrangement – that the scientific image of water exhausts its nature and the manifest image is otiose.

To be sure, the chemical facts are actually a bit more complicated than the routine reference to water as H2O would imply (Chang 2012, pp.xvi and 210; Hoffmann 1995, pp. 32-34; Van Brakel 2000, pp. 80-81), but that is not the main point. As Van Brakel points out, when dealing with water we judge that we are dealing with a substance, and with the same substance over time, from its macroscopic properties and apart from our knowledge of chemistry. For example, we don’t need to know anything about the microstructure to judge that the water in a certain glass that is first liquid, then freezes, then melts and is liquid again, is the same stuff throughout. We don’t need to know the microstructure to judge that when we stir sand into a glass of water and then filter it out again, we haven’t altered the nature of the stuff in the glass but just temporarily made it dirty. And so on. We have a rough and ready “manifest image” conception of what water is, and this guides us when we investigate the microstructure. It is that stuff, specifically, the stuff that counts as water in the manifest image, that the scientific image is describing when it makes reference to H2O. It is only because a certain part of nature has first been carved off in the manifest image and labeled “water” that chemistry can go on to investigate it. “[S]uch scientific explanations refer to water, where the latter term refers to manifest water (the same water Aristotle speculated about)... [T]he manifest image determines which micro-essences are to be selected” (Van Brakel 2000, pp. 78 and 81).

The reductionist might respond that it is chemistry that explains the manifest image attributes, in terms of a microstructure of molecules and their interrelationships. Once we have this microstructural description, he might claim, we can account for the nature and identity of water without making reference to the manifest image description that initially guided our investigation. But as Van Brakel argues, this will not work (2000, p. 79). Do the identity criteria for the water in the glass in our example include the velocity and relative positions of the molecules? If so, then the reductionist cannot account for the fact that the same water persists over time, since the velocities and relative positions are constantly changing. By contrast, the non-reductionist has no problem, because the macroscopic features of the glass of water persist despite these changes at the micro-level. Suppose, then, that the reductionist says that velocity and relative position are not part of the identity criteria. Then he will be unable to appeal to a feature such as temperature as a mark of identity or difference, since the molecular motion that is the micro-level correlate of temperature is constantly changing. Again, the non-reductionist has no problem here, because these variations average out at the macro-level.

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Return to Lesson 7: Scientific Evidence Against Reductionism