Finitism is a philosophy of mathematics that accepts the existence only of finite mathematical objects. The finitist philosophy of mathematics is best understood in comparison to the mainstream philosophy of mathematics where infinite mathematical objects like infinite sets are accepted as legitimate mathematical objects existing in the Platonic universe of mathematics.
The main idea of finististic mathematics is not accepting the existence of infinite objects like infinite sets. While all natural numbers are accepted as existing, the set of all natural numbers is not considered to exist as a mathematical object. Therefore quantification over infinite domains is not considered meaningful. The mathematical theory often associated with finitism is Thoralf Skolem's primitive recursive arithmetic.
The introduction of infinite mathematical objects was a development in mathematics that occurred a few centuries ago. The use of infinite objects was a controversial topic among mathematicians. The issue entered a new phase when Georg Cantor, starting in 1874, introduced what is now called naive set theory and used it as a base for his work on transfinite numbers. When paradoxes such as Russell's paradox, Berry's paradox and the Burali-Forti paradox were discovered in Cantor's naive set theory, the issue became a heated topic among mathematicians.
There were various positions taken by mathematicians. All agreed about finite mathematical objects like natural numbers. However there were disagreements regarding infinite mathematical objects. One position was the intuitionistic mathematics that was advocated by L.E.J. Brouwer, which rejected the existence of infinite objects until they are constructed.
Another position was endorsed by David Hilbert: finite mathematical objects are concrete objects, infinite mathematical objects are ideal objects, and accepting ideal mathematical objects does not cause a problem regarding finite mathematical objects. More formally, Hilbert believed that it is possible to show that any theorem about finite mathematical objects that can be obtained using ideal infinite objects can be also obtained without them. Therefore allowing infinite mathematical objects would not cause a problem regarding finite objects. This led to Hilbert's program of proving consistency of set theory using finitistic means as this would imply that adding ideal mathematical objects is conservative over the finitistic part. Hilbert's views are also associated with formalist philosophy of mathematics. Hilbert's goal of proving the consistency of set theory or even arithmetic through finitistic means turned out to be an impossible task due to Kurt Gödel's incompleteness theorems. However, by Harvey Friedman's grand conjecture most mathematical results should be provable using finitistic means.
Hilbert did not give a rigorous explanation of what he considered finitistic and refer to as elementary. However, based on his work with Paul Bernays some experts like William Tait have argued that the primitive recursive arithmetic can be considered as an upper bound on what Hilbert considered as finitistic mathematics.
In the years following Gödel's theorems, as it became clear that there is no hope of proving consistency of mathematics, and with development of axiomatic set theories like Zermelo–Fraenkel set theory and the lack of any evidence against its consistency, most mathematicians lost interest in the topic. Today most classical mathematicians are considered Platonist and believe in the existence of infinite mathematical objects and a set-theoretical universe.
Classical finitism vs. strict finitism
In her book Philosophy of Set Theory, Mary Tiles characterized those who allow countably infinite objects as classical finitists, and those who do not allow countably infinite objects as strict finitists. Historically, the written history of mathematics was thus classically finitist until Cantor discovered the hierarchy of transfinite cardinals in the end of the 19th century.
Views regarding infinite mathematical objects
God created the natural numbers, all else is the work of man.
Although he denied it, much of Ludwig Wittgenstein's writing on mathematics has a strong affinity with finitism.
If finitists are contrasted with transfinitists (proponents of e.g. Georg Cantor's hierarchy of infinities), then also Aristotle may be characterized as a strict finitist. Aristotle especially promoted the potential infinity as a middle option between strict finitism and actual infinity. (Note that Aristotle's actual infinity means simply an actualization of something never-ending in nature, when in contrast the Cantorist actual infinity means the transfinite cardinal and ordinal numbers, that have nothing to do with the things in nature):
But on the other hand to suppose that the infinite does not exist in any way leads obviously to many impossible consequences: there will be a beginning and end of time, a magnitude will not be divisible into magnitudes, number will not be infinite. If, then, in view of the above considerations, neither alternative seems possible, an arbiter must be called in.—Aristotle, Physics, Book 3, Chapter 6
Ultrafinitism (also known as ultraintuitionism) has an even more conservative attitude towards mathematical objects than finitism, and has objections to the existence of finite mathematical objects when they are too large.
- Eriksson, K., Estep D., and Johnson C. Applied Mathematics: Body and Soul. Volume 1. Springer, 2004, p. 230-232.
- From an 1886 lecture at the 'Berliner Naturforscher-Versammlung', according to H. M. Weber's memorial article, Leopold Kronecker, in Jahresbericht der Deutschen Mathematiker-Vereinigung, Vol. 2 1891-92