Mathematical anti-Platonism
Many philosophers cannot bring themselves to believe in abstract objects. However, there are not many tenable alternatives to mathematical Platonism. One option is to maintain that there do exist such things as numbers and sets (and that mathematical theorems provide true descriptions of these things) while denying that these things are abstract objects. Views of this kind can be called realistic versions of anti-Platonism. Like Platonism, they are still versions of mathematical realism because they maintain that mathematical theorems provide true descriptions of some part of the world.
In contrast to realistic versions of anti-Platonism, there is also an antirealist view known as mathematical nominalism. This view rejects the belief in the existence of numbers, sets, and so on and also rejects the belief that mathematical theorems provide true descriptions of some part of the world.
The two main alternatives to Platonism, then, are realistic anti-Platonism and nominalism. These alternatives are described more fully in the following two sections.
Realistic anti-Platonism
There are two different versions of realistic anti-Platonism, namely, psychologism and physicalism. Psychologism is the view that mathematical theorems are about concrete mental objects of some sort. In this view, numbers and circles and so on do exist, but they do not exist independently of people; instead, they are concrete mental objects—in particular, ideas in people’s heads. As will become clearer below (in the section Mathematical Platonism: for and against), psychologism has serious problems and is no longer endorsed by many philosophers; nonetheless, it was popular during the late 19th and early 20th centuries, the most notable proponents being the German philosopher Edmund Husserl and the Dutch mathematicians L.E.J. Brouwer and Arend Heyting.
Physicalism, on the other hand, is the view that mathematics is about concrete physical objects of some sort. Advocates of this view agree with Platonists that there exist such things as numbers and sets, and, unlike adherents of psychologism, they also agree that these things exist independently of people and their thoughts. Physicalists differ from Platonists, however, in holding that mathematics is about ordinary physical objects. There are a few different versions of this view. For example, one might hold that geometric objects, such as circles, are regions of actual physical space. Similarly, sets might be claimed to be piles of actual physical objects—thus, a set of eggs would be nothing more than the aggregate of physical matter that makes up the eggs. Moving on to numbers, one strategy is to take them to be physical properties of some sort—for example, properties of piles of physical objects, so that, for instance, the number 3 might be a property of a pile of three eggs. It should be noted here that many people have endorsed a Platonistic view of properties. In particular, Plato thought that, in addition to all the red things he observed in the world, there exists an independent property of redness and that this property was an abstract object. Aristotle, on the other hand, thought that properties exist in the physical world; thus, in his view, redness exists in particular objects, such as red houses and red apples, rather than as an abstract object outside of space and time. So in order to motivate a physicalistic view of mathematics by claiming that numbers are properties, one would also have to argue for an Aristotelian, or physicalistic, view of properties. One person who has developed a view of this sort since Aristotle is the Australian philosopher David Armstrong.
Another strategy for interpreting talk of numbers to be about the physical world is to interpret it as talk about actual piles of physical objects rather than properties of such piles. For instance, one might maintain that the sentence “2 + 3 = 5” is not really about specific entities (the numbers 2, 3, and 5); rather, it says that whenever a pile of two objects is pushed together with a pile of three objects, the result is a pile of five objects. A view of this sort was developed by the English philosopher John Stuart Mill in the 19th century.