Especially in the 14th century, controversies associated with the opposition between nominalism and realism greatly exercised philosophers and theologians in the Latin West. These terms have been been variously understood, but as a first approximation, nominalism wants to deny claims about the real basis of abstractions that the realism of this context wants to affirm.
In this case, a polar opposition is concealed behind a pair of concrete terms (nominalism, realism), where in context one is understood as the simple negation of the other. As usual with debates around distinctions based on polar opposition rather than more limited and definite determinate negation, the greatest interest often lies in the way each side tries to recover something like the strong points of the other side, but in its own terms.
These controversies are worth lingering over for several reasons. For one thing, they help illustrate the great diversity, subtlety, and liveliness of medieval thought. For another, they develop many fine distinctions that are of lasting value in talking about human knowledge and understanding. We would all like to rightly apprehend things, whatever that means. The waters are commonly muddied not only by insufficient distinctions among things, but also by fundamental unclarity or ambiguity on the meaning of “existence” or “reality”, which gets worse where abstract things are involved. Who we might think was right in the debates is of secondary importance compared to clarifications of this kind. Finally, these debates involved much discussion of mental representation, its origins, and its role in thought.
Speaking with very broad brush, nominalism begins as a critique of a sort of “platonism”. Such platonism wants to say the universal is more real than the particular. It may go on to claim that abstract entities are as real as — or more real than — concrete ones. It may extend to further claims that universals simply “exist” in some pure way, independent of space and time. Nominalism in general wants to say the opposite, that universals are actually not real at all.
Aristotle already criticized platonist views of the sort just mentioned, while still maintaining that the development of universals is essential to knowledge. I think that in the big picture, he wanted to recommend an essentially even-handed approach, recognizing both universals and particulars as necessary to any developed view of experience, while pointing out their very different and complementary roles. Whatever we may think about the reality or unreality or existence or nonexistence of given things or of various kinds of things, we need universals to support the implicit reasoning standing behind any developed knowledge. We also need particulars as practical starting points, and as cross-checks to keep us honest. This does not yet make any claim about reality or existence that might support such needs. Aristotle often practiced a careful minimalism, sticking to essentials and leaving other questions open, and this is a good case in point.
Albert the Great and Thomas Aquinas wanted to develop Aristotle’s position into a firmer doctrine, classically called moderate realism. Most people agree that Aristotle thought universals do not “exist” independently of particular things and thought. Albert and Thomas argued that implicitly, what Aristotle said committed him to 1) a claim that universals are real and 2) a claim that universals exist, but only in concrete things and in thought.
Nominalists especially disputed the claim that universals exist in concrete things. They most commonly advocated a mental origin of universals, while differing on the precise status attributed to them. Already in the 12th century, Roscellinus had argued that universals are mere names (root of the word “nominalism”). Whether or not the great Peter Abelard should be interpreted as a nominalist or a middle-of-the road “conceptualist” is contested among scholars.
The theologian William of Occam (1285 – 1347) was the most famous medieval nominalist. Early in his career, he argued that universals were ficta (“fictions”) of the mind. Later, he worried that this still tacitly presupposed they were representations, which would seem to still imply something corresponding to them in external objects. He then argued that external objects have causal impact on the mind, but not by representation.
The important secular master John Buridan (1301 – 1358) is usually also called a nominalist. Buridan was one of the leading logicians of the middle ages, and wrote on a wide range of philosophical questions. He had several noteworthy students who are also considered nominalists, including the logician, natural philosopher, and bishop Albert of Saxony (1320 – 1390). Marsilius of Inghen (1340 – 1396) was another nominalist who wrote on logic, natural philosophy, and theology. The theologian Gregory of Rimini (1300 – 1358) is also considered a nominalist.
The great theologian John Duns Scotus (1265 – 1308) was a commited realist who nonetheless influenced Occam on some relatively unrelated points. The influential Walter Burley (1275 – 1344) is sometimes called an extreme realist. Paul of Venice (1369 – 1429) was formerly classed as a nominalist, but is now considered a realist.
Among those who were called nominalists, there were many different views and distinctions related to the complex medieval theories of sensible and intelligible “species”. In one aspect, these were mental representations, but theories of sensible species usually had a physical component loosely inspired by Stoicism. Occam denied species, while Buridan made use of them.
From the 12th century onward, Latin philosophers developed sophisticated original theories of the different kinds of “supposition”, or generic ways in which something said can be meant. The general notion was that the kind of supposition that should be read into a concrete utterance should be determined by analyzing the context of the utterance in various ways. This was basically a kind of semantics. What is perhaps surprising is that broadly similar supposition theories were largely shared by dedicated nominalists like Occam and commited realists like Walter Burley, providing a common vocabulary.
On a side note, Occam’s causal impact theory seems problematic from the point of view of the development here. While its avoidance of dependence on representation is attractive, a direct causal link from external objects to thoughts does not seem adequate to account for the full range of diversity of thoughts. Also, there seems to be an incipient mentalism already at work here, related to that of Avicenna.
Occam was a theological voluntarist and a fideist. Fideism is the belief that faith offers a kind of knowledge superior to reason, an extreme position that was repeatedly condemned by the Church. Occam has nonetheless often been named as a major precursor of the point of view of modern science. Even though some connections can be made, this seems questionable as well, given his mainly theological intent and the character of the theology he promoted.