[Because I am about to go on holiday, I am cheating a bit this week. Below an excerpt from my manuscript-in-progress-on Adam Smith.--ES]
It is evident that the mind takes pleasure in observing the resemblances that are discoverable betwixt different objects. It is by means of such observations that it endeavours to arrange and methodise all its ideas, and to reduce them into proper classes and assortments. . . When two objects, however unlike, have often been observed to follow each other, and have constantly presented themselves to the senses in that order, they come to be so connected together in the fancy, that the idea of the one seems, of its own accord, to call up and introduce that of the other. If the objects are still observed to succeed each other as before, this connection, or, as it has been called, this association of their ideas, becomes stricter and stricter, and the habit of the imagination to pass from the conception of the one to that of the other, grows more and more rivetted and confirmed. As its ideas move more rapidly than external objects, it is continually running before them, and therefore anticipates, before it happens, every event which falls out according to this ordinary course of things.--Adam Smith (History of Astronomy, 2.1-7, 37-41)
At NewAPPS I have posted about this passage from Smith's posthumously published (1795) Essays on Philosophical Subjects (EPS) before. Smithian causation is founded on a pleasing psychological disposition that activates the mind to notice and, perhaps, even search out resemblances between objects. In the most Humean part of Smith’s treatment above, the habitual observation of the ordered, constant conjunction of two objects causes the habitual conjoining of the two ideas in order to form an association between them (as Smith says) in the imagination. (In Hume’s terminology this is called a "natural relation.")
According to Smith this pleasing activity of classifying ideas leads into what we may call a “natural taxonomy” including abstract categories (“proper classes and assortment”). To be clear: Smith views on the origin of a natural taxonomy do not commit him to claiming that such a natural taxonomy is true. In particular our tendency toward natural taxonomy is at the root of the long-popular Aristotelian metaphysical system. In Smith's (unjustly neglected) "History of Ancient Logics and Metaphysics," Smith explains how many of the “doctrines” of Aristotle seem “to have arisen, more from the nature of language, than from the nature of things.” (EPS 125; recall also this post on Tim Maudlin & Judith Butler). While Smith is not harshly critical of Aristotle's system (“With all its imperfections it was excusable, in the beginnings of philosophy, and is not a great deal more remote from the truth, than many others which have since been substituted in its room by some of the greatest pretenders to accuracy and precision,” (EPS 125)), he does not endorse it either.
Now, according to Smith we naturally group things together by way of common, sensible qualities. In the nineteenth century this leads to two, conflicting interpretations of Smith’s metaphysics. Sir William Hamilton treats Smith as a nominalist successor to Berkeley and Hume (Hamilton 1861: 474). In his argument, Hamilton relies exclusively on the following passage from Smith's essay on the "Origin of Languages" (which was originally appended to the third edition of The Theory of Moral Sentiments by Smith [it's a great scandal this was removed in the so-called Glasgow Edition!]):
It is this application of the name of an individual to a great multitude of objects, whose resemblance naturally recalls the idea of that individual, and of the name which expresses it, that seems originally to have given occasion to the formation of those classes and assortments, which, in the schools, are called genera and species, and of which the ingenious and eloquent M. Rousseau of Geneva finds himself so much at a loss to account for the origin. (Languages 2, 204-205)
In order to explain our tendency toward a natural (but not necessarily true) taxonomy, Smith draws here on a mental mechanism defended by Hume (and attributed by Hume to Berkeley): “all general ideas are nothing but particular ones, annexed to a certain term, which gives them a more extensive signification, and makes them recall upon occasion other individuals, which are similar to them.” (Treatise 22.214.171.124) This mechanism has become known as the “revival set” in Hume scholarship (Garrett 24ff). So, Hamilton’s interpretation seems well founded.
Even so, we can find a more thorough alternative to Hamilton’s interpretation earlier in the nineteenth century proposed by Hamilton’s teacher, Dugald Stewart (Smith’s first biographer), who treats Smith’s account approvingly and quotes the following passage from Smith's "History of Astronomy" (2.1):
Where it can observe but one single quality, that is common to a great variety of otherwise widely different objects, that single circumstance will be sufficient for it to connect them all together, to reduce them to one common class, and to call them by one general name. It is thus that all things endowed with a power of self-motion, beasts, birds, fishes, insects, are classed under the general name of Animal; and that these again, along with those which want that power, are arranged under the still more general word Substance: and this is the origin of those assortments of objects and ideas which in the schools are called Genera and Species, and of those abstract and general names, which in all languages are made use of to express them. (EPS 38)
Stewart calls attention to two features of Smith’s treatment later overlooked by Hamilton. First, Smith is describing a mental mechanism which allows us to see one aspect of an object “apart from the rest.” When we attend to particular sensible qualities, we isolate them from others. Second, this mechanism is properly understood as the power of abstraction. (Stewart: 132) The first claim is uncontroversial and compatible with a nominalist interpretation. But the second feature is controversial and potentially at odds with the nominalist interpretation of Smith. Moreover, Stewart locates this power of abstraction not in the imagination, but in the understanding (without specifically attributing this further claim to Smith). Now Smith is familiar with such a philosophy of mind because in reflecting on Plato's and Aristotle’s treatment of so-called “specific essences,” Smith describes a similar mechanism as follows: “Mankind have had, at all times, a strong propensity to realize their own abstractions.” (EPS 125; see also this fascinating footnote.)
I am inclined to agree with Stewart’s interpretation, then, that according to Smith our natural taxonomy is itself founded on the original propensity toward abstraction. After all, in his essay on the "Origin of Languages" Smith had insisted that “The man who first distinguished a particular object by the epithet of green, must have observed other objects that were not green, from which he meant to separate it by this appellation. The institution of this name, therefore, supposes comparison. It likewise supposes some degree of abstraction.” (Languages) Even in cases of straightforward naming we rely to some degree on abstraction. But hereby Smith has inverted the Berkeley-Hume position! Rather than using the “revival set” mechanism as a way to explain away abstract ideas, Smith suggest that this mechanism itself crucially relies on abstraction. Of course, it does not follow from this that Smith is not a nominalist, but to settle that question we will need to look at his broader metaphysics on some other occassion.
There is an important moral here that applies to all of Smith’s writings. Often Smith works with Humean concepts and mechanisms, but in his discussion he frequently subverts these in subtle ways (see Fleischacker 2012); Smith does so sometimes to strengthen Hume’s main point, sometimes to radicalize Hume’s aims, and sometimes to offer a dramatically different account. So, even when one notices Humean commitments in Smith one has to be cautious about conflating their positions.