In
Philosophy of mathematics, the concept of "mathematical objects" touches on topics of
existence,
identity, and the
nature of
reality.[2] In
metaphysics, objects are often considered
entities that possess
properties and can stand in various
relations to one another.[3] Philosophers debate whether mathematical objects have an independent existence outside of
human thought (
realism), or if their existence is dependent on mental constructs or language (
idealism and
nominalism). Objects can range from the
concrete: such as
physical objects usually studied in
applied mathematics, to the
abstract, studied in
pure mathematics. What constitutes an "object" is foundational to many areas of philosophy, from
ontology (the study of being) to
epistemology (the study of knowledge). In mathematics, objects are often seen as entities that exist independently of the
physical world, raising questions about their ontological status.[4][5] There are varying
schools of thought which offer different perspectives on the matter, and many famous mathematicians and philosophers each have differing opinions on which is more correct.[6]
Plato: The ancient
Greek philosopher who, though not a mathematician, laid the groundwork for Platonism by positing the existence of an abstract realm of perfect
forms or ideas, which influenced later thinkers in mathematics.
Kurt Gödel: A 20th-century logician and mathematician, Gödel was a strong proponent of mathematical Platonism, and his work in
model theory was a major influence on
modern platonism
Roger Penrose: A contemporary
mathematical physicist, Penrose has argued for a Platonic view of mathematics, suggesting that mathematical truths exist in a realm of abstract reality that we discover.[12]
Nominalism
Nominalism denies the independent existence of mathematical objects. Instead, it suggests that they are merely
convenient fictions or shorthand for describing relationships and structures within our language and theories. Under this view, mathematical objects don't have an existence beyond the symbols and concepts we use.[13][14]
Some notable nominalists incluse:
Nelson Goodman: A philosopher known for his work in the philosophy of science and nominalism. He argued against the existence of abstract objects, proposing instead that mathematical objects are merely a product of our linguistic and symbolic conventions.
Hartry Field: A
contemporary philosopher who has developed the form of nominalism called "
fictionalism," which argues that mathematical statements are useful fictions that don't correspond to any actual abstract objects.[15]
Logicism
Logicism asserts that all mathematical truths can be reduced to
logical truths, and all objects forming the subject matter of those branches of mathematics are logical objects. In other words, mathematics is fundamentally a branch of
logic, and all mathematical concepts,
theorems, and truths can be derived from purely logical principles and definitions. Logicism faced challenges, particularly with the Russillian axioms, the Multiplicative axiom (now called the
Axiom of Choice) and his
Axiom of Infinity, and later with the discovery of
Gödel's incompleteness theorems, which showed that any sufficiently powerful
formal system (like those used to express
arithmetic) cannot be both
complete and
consistent. This meant that not all mathematical truths could be derived purely from a logical system, undermining the logicist program.[16]
Some notable logicists include:
Gottlob Frege: Frege is often regarded as the founder of logicism. In his work, Grundgesetze der Arithmetik (Basic Laws of Arithmetic), Frege attempted to show that arithmetic could be derived from logical axioms. He developed a formal system that aimed to express all of arithmetic in terms of logic. Frege's work laid the groundwork for much of
modern logic and was highly influential, though it encountered difficulties, most notably
Russell's paradox, which revealed inconsistencies in Frege's system.[17]
Bertrand Russell: Russell, along with
Alfred North Whitehead, further developed logicism in their monumental work Principia Mathematica. They attempted to derive all of mathematics from a set of
logical axioms, using a
type theory to avoid the paradoxes that Frege's system encountered. Although Principia Mathematica was enormously influential, the effort to reduce all of mathematics to logic was ultimately seen as incomplete. However, it did advance the development of
mathematical logic and
analytic philosophy.[18]
Formalism
Mathematical formalism treats objects as symbols within a
formal system. The focus is on the manipulation of these symbols according to specified rules, rather than on the objects themselves. One common understanding of formalism takes mathematics as not a body of propositions representing an abstract piece of reality but much more akin to a game, bringing with it no more ontological commitment of objects or properties than playing
ludo or
chess. In this view, mathematics is about the consistency of formal systems rather than the discovery of pre-existing objects. Some philosophers consider logicism to be a type of formalism.[19]
Some notable formalists include:
David Hilbert: A leading mathematician of the early 20th century, Hilbert is one of the most prominent advocates of formalism. He believed that mathematics is a system of formal rules and that its truth lies in the consistency of these rules rather than any connection to an abstract reality.[20]
Hermann Weyl: German mathematician and philosopher who, while not strictly a formalist, contributed to formalist ideas, particularly in his work on the foundations of mathematics.[21]
Constructivism
Mathematical constructivism asserts that it is necessary to find (or "construct") a specific example of a mathematical object in order to prove that an example exists. Contrastingly, in classical mathematics, one can prove the existence of a mathematical object without "finding" that object explicitly, by assuming its non-existence and then deriving a
contradiction from that assumption. Such a
proof by contradiction might be called non-constructive, and a constructivist might reject it. The constructive viewpoint involves a verificational interpretation of the
existential quantifier, which is at odds with its classical interpretation.[22] There are many forms of constructivism.[23] These include the program of
intuitionism founded by
Brouwer, the
finitism of
Hilbert and
Bernays, the constructive recursive mathematics of mathematicians
Shanin and
Markov, and
Bishop's program of
constructive analysis.[24] Constructivism also includes the study of
constructive set theories such as
Constructive Zermelo–Fraenkel and the study of philosophy.
Structuralism
Structuralism suggests that mathematical objects are defined by their place within a structure or system. The nature of a number, for example, is not tied to any particular thing, but to its role within the system of
arithmetic. In a sense, the thesis is that mathematical objects (if there are such objects) simply have no intrinsic nature.[25][26]
Some notable structuralists include:
Paul Benacerraf: A philosopher known for his work in the philosophy of mathematics, particularly his paper "What Numbers Could Not Be," which argues for a structuralist view of mathematical objects.
Stewart Shapiro: Another prominent philosopher who has developed and defended structuralism, especially in his book Philosophy of Mathematics: Structure and Ontology.[27]
Objects versus mappings
Frege famously distinguished between functions and objects.[29] According to his view, a function is a kind of ‘incomplete’
entity that
maps arguments to values, and is denoted by an incomplete expression, whereas an object is a ‘complete’ entity and can be denoted by a singular term. Frege reduced
properties and
relations to functions and so these entities are not included among the objects. Some authors make use of Frege's notion of ‘object’ when discussing abstract objects.[30] But though Frege's sense of ‘object’ is important, it is not the only way to use the term. Other philosophers include properties and relations among the abstract objects. And when the background context for discussing objects is
type theory, properties and relations of higher type (e.g., properties of properties, and properties of relations) may be all be considered ‘objects’. This latter use of ‘object’ is interchangeable with ‘entity.’ It is this more broad interpretation that mathematicians mean when they use the term 'object'.[31]
^Rettler, Bradley; Bailey, Andrew M. (2024),
"Object", in Zalta, Edward N.; Nodelman, Uri (eds.), The Stanford Encyclopedia of Philosophy (Summer 2024 ed.), Metaphysics Research Lab, Stanford University, retrieved 2024-08-28
^Carroll, John W.; Markosian, Ned (2010). An introduction to metaphysics. Cambridge introductions to philosophy (1. publ ed.). Cambridge: Cambridge University Press.
ISBN978-0-521-82629-7.
^Falguera, José L.; Martínez-Vidal, Concha; Rosen, Gideon (2022),
"Abstract Objects", in Zalta, Edward N. (ed.), The Stanford Encyclopedia of Philosophy (Summer 2022 ed.), Metaphysics Research Lab, Stanford University, retrieved 2024-08-28
^Horsten, Leon (2023),
"Philosophy of Mathematics", in Zalta, Edward N.; Nodelman, Uri (eds.), The Stanford Encyclopedia of Philosophy (Winter 2023 ed.), Metaphysics Research Lab, Stanford University, retrieved 2024-08-29
^Colyvan, Mark (2024),
"Indispensability Arguments in the Philosophy of Mathematics", in Zalta, Edward N.; Nodelman, Uri (eds.), The Stanford Encyclopedia of Philosophy (Summer 2024 ed.), Metaphysics Research Lab, Stanford University, retrieved 2024-08-28
^Paseau, Alexander (2016),
"Naturalism in the Philosophy of Mathematics", in Zalta, Edward N. (ed.), The Stanford Encyclopedia of Philosophy (Winter 2016 ed.), Metaphysics Research Lab, Stanford University, retrieved 2024-08-28
^Horsten, Leon (2023),
"Philosophy of Mathematics", in Zalta, Edward N.; Nodelman, Uri (eds.), The Stanford Encyclopedia of Philosophy (Winter 2023 ed.), Metaphysics Research Lab, Stanford University, retrieved 2024-08-28
^Linnebo, Øystein (2024),
"Platonism in the Philosophy of Mathematics", in Zalta, Edward N.; Nodelman, Uri (eds.), The Stanford Encyclopedia of Philosophy (Summer 2024 ed.), Metaphysics Research Lab, Stanford University, retrieved 2024-08-27
^Roibu, Tib (2023-07-11).
"Sir Roger Penrose". Geometry Matters. Retrieved 2024-08-27.
^Bueno, Otávio (2020),
"Nominalism in the Philosophy of Mathematics", in Zalta, Edward N. (ed.), The Stanford Encyclopedia of Philosophy (Fall 2020 ed.), Metaphysics Research Lab, Stanford University, retrieved 2024-08-27
^Tennant, Neil (2023),
"Logicism and Neologicism", in Zalta, Edward N.; Nodelman, Uri (eds.), The Stanford Encyclopedia of Philosophy (Winter 2023 ed.), Metaphysics Research Lab, Stanford University, retrieved 2024-08-27
^Weir, Alan (2024),
"Formalism in the Philosophy of Mathematics", in Zalta, Edward N.; Nodelman, Uri (eds.), The Stanford Encyclopedia of Philosophy (Spring 2024 ed.), Metaphysics Research Lab, Stanford University, retrieved 2024-08-28
^Bell, John L.; Korté, Herbert (2024),
"Hermann Weyl", in Zalta, Edward N.; Nodelman, Uri (eds.), The Stanford Encyclopedia of Philosophy (Summer 2024 ed.), Metaphysics Research Lab, Stanford University, retrieved 2024-08-28
^Bridges, Douglas; Palmgren, Erik; Ishihara, Hajime (2022),
"Constructive Mathematics", in Zalta, Edward N.; Nodelman, Uri (eds.), The Stanford Encyclopedia of Philosophy (Fall 2022 ed.), Metaphysics Research Lab, Stanford University, retrieved 2024-08-28
^Troelstra, Anne Sjerp (1977a). "Aspects of Constructive Mathematics". Handbook of Mathematical Logic. 90: 973–1052.
doi:10.1016/S0049-237X(08)71127-3
^Reck, Erich; Schiemer, Georg (2023),
"Structuralism in the Philosophy of Mathematics", in Zalta, Edward N.; Nodelman, Uri (eds.), The Stanford Encyclopedia of Philosophy (Spring 2023 ed.), Metaphysics Research Lab, Stanford University, retrieved 2024-08-28
^Philosophy of Mathematics: Structure and Ontology. Oxford University Press, 1997.
ISBN0-19-513930-5
^Falguera, José L.; Martínez-Vidal, Concha; Rosen, Gideon (2022),
"Abstract Objects", in Zalta, Edward N. (ed.), The Stanford Encyclopedia of Philosophy (Summer 2022 ed.), Metaphysics Research Lab, Stanford University, retrieved 2024-08-28
Further reading
Azzouni, J., 1994. Metaphysical Myths, Mathematical Practice. Cambridge University Press.
Burgess, John, and Rosen, Gideon, 1997. A Subject with No Object. Oxford Univ. Press.
Hersh, Reuben, 1997. What is Mathematics, Really? Oxford University Press.
Sfard, A., 2000, "Symbolizing mathematical reality into being, Or how mathematical discourse and mathematical objects create each other," in Cobb, P., et al., Symbolizing and communicating in mathematics classrooms: Perspectives on discourse, tools and instructional design. Lawrence Erlbaum.
Stewart Shapiro, 2000. Thinking about mathematics: The philosophy of mathematics. Oxford University Press.