I wish to address the topic of realism vs. scholastic realism vs. nominalism. I hope that it will further clarify my earlier comments on the subject both here and at Footnotes2Plato, Knowledge Ecology, and After Nature. The remainder of my discussion should not be construed as a continuance of that conversation, but that it was the occasion for the present thought.
“Realism” is usually short for “realism about the external world” that is opposed to “idealism.” The former supposes that the world exists independently from mind, while the latter supposes that mind is fundamental to existence. Idealism is closely associated with “rationalism,” which holds that rationality or intellect is fundamental to existence. Given that mind implies rationalism—at least in a historic western context—rationalism implies idealism. “Realism” is commonly affirmed by those who merely assert the existence of the external world, and that was how it was defined by Levi Bryant in the mentioned conversations, who affirmed realism and (epistemic?) nominalism.
Suppose that one is a metaphysical nominalist, e.g., a Heraclitus or Thomas Hobbes. This kind of nominalist asserts that things do not have a discrete identity, e.g., denies either the reality or application of the logical law of identity. To deny the reality of the law is to say that there is no identity to anything, e.g., all is flux. Heraclitus might be said to approach this view, as the only unities within his metaphysics are the fact of flux and the persistence of the upward and downward way, e.g., change and logos. One could also deny the effective applicability of the law, e.g., arguing a Hobbesian corpuscular metaphysics in which body and motion are the primitives. In this case, nothing has an essence, even though diversity might be real in the sense that different configurations of bodies and forces may have different effects. Although neither of these examples is perfect, they do give an idea of the problems and common historic variants of metaphysical nominalism.
Suppose that one is an epistemic nominalist, e.g., a David Hume. This kind of nominalist does not deny (metaphysical) identity, but does deny that human beings can know it. Hume is an excellent example with which to move forward, because he is also an empiricist, and is thus a kind of nominalist that we are likely to see today, unlike Heraclitus and Hobbes. The combination of empiricism and epistemic nominalism birth a conundrum for such a thinker who also wishes to do metaphysics. Metaphysics becomes impossible. Barring exotic re-definitions and qualifications, this person has denied all the intellectual tools in which to do metaphysics. That is, an empiricist-nominalist is limited to immediate experience, because such a person cannot unequivocally name anything beyond experience. If the person equivocally names something, then the naming has nothing to do with the (metaphysical) identity of the thing. There are still options, for example, in pragmatic moves that redefine truth in terms of anticipated consequences. However, such options still court Hume’s inductive fallacy of assuming that the past will be like the future, which becomes a suffocating limitation for a nominalist.
The problem with nominalism is that one berefts oneself of the intellectual tools to do metaphysics. It is not impossible, but it greatly limits what positions one can claim without begging the question and becoming incoherent. If one is an epistemic nominalist and an empiricist, one must defend, to an extreme degree, one’s ability to perform metaphysics. You affirm that you experience something, but you know not what. Even if one asserts realism (about the external world) along with nominalism, this gets that person little, because the person admits that reality cannot be known in principle. Perhaps the best that person can do is affirm a Jamesian pragmatic principle about the “cash value” of holding this-or-that metaphysical theory. That is a defensible hypothesis, but a pragmatic abduction requires that one should that one’s metaphysics leads to actual change in the world. A Jamesian pragmatic hypothesis substitutes a traditional truth claim with an empirical and consequential one, and that metaphysician better like working with one’s hands. It’s a change of intellectual venue entirely, and cannot be purely theoretically justified. I say this because it’s too easy to claim Jamesian pragmatism and then overlook his verification requirement—having one’s cake and eating it too. (Note that James’ view does not hold for the other pragmatists, which is something often overlooked, but the other pragmatists were unequivocal realists vs. James’ ambiguity at points.)
Metaphysical nominalism is opposed to scholastic realism per the reality of universals. If one affirms the reality of universals, i.e., the reality of descriptive categories (cf Aristotle’s Categories), then one is much empowered. One need no longer perform the contradiction of claiming to do metaphysics without the descriptive categories to do so. When claiming the reality of universals, by the way, one is not claiming that (human) conceptual categories match the ontological categories. One claims that there are ontological categories towards which our conceptual categories might aim, i.e., that unity in the universe is real. In contrast, recall that Kant said that unity (or rules) are a feature of the (human) understanding, and therefore at best metaphysics extends little past sensibility. That is precisely what being a realist about universals denies. Moreover, if one is a process metaphysician rather than a substance metaphysician, another option is opened.
Process metaphysicians following in the wake of C.S. Peirce, who affirm the reality of continuity, may rest assured that there is a continuity of the ontological categories and the phenomenological and conceptual categories. That is, to leap a few steps ahead, experience is real. We experience things in their immediacy, and not through some annihilating Kantian mediation. However, I did not say that the experienced quality is isomorphic to the thing itself, because what also frequently comes with this view is non-representation theories of experience. Human experience is a semiotic process by which we interact with the thing and encode part of the interaction as consciousness. Sorry, no mirroring of nature here, despite being twice-over a realism and a naturalism.
I am, of course, describing in my own way the pragmatist position. Its empiricism is too often mistaken for a Lockean empiricism rather than a Hegelian empiricism; i.e., experience is a mediated immediacy. Following Peirce, phenomenologically-speaking, the mediation is physico-semiotic.
I hope that this post has been informative. My intent was to further delineate the various forms of nominalism and give some historic examples, while explaining my own interest in the topic. I have more to say that I will likely add in the comments.