''A posteriori'' necessity is a thesis in

metaphysics Metaphysics is the branch of philosophy that studies the fundamental nature of reality, the first principles of being, identity and change, space and time, causality, necessity, and possibility. It includes questions about the nature of conscio ...

and the

philosophy of language In analytic philosophy, philosophy of language investigates the nature of language and the relations between language, language users, and the world. Investigations may include inquiry into the nature of meaning, intentionality, reference, ...

, that some statements of which we must acquire knowledge

a posteriori ("from the earlier") and ("from the later") are Latin phrases used in philosophy to distinguish types of knowledge, justification, or argument by their reliance on empirical evidence or experience. knowledge is independent from current ex ...

are also

necessarily true Logical truth is one of the most fundamental concepts in logic. Broadly speaking, a logical truth is a statement which is true regardless of the truth or falsity of its constituent propositions. In other words, a logical truth is a statement whic ...

. It challenges previously widespread belief that only ''a priori'' knowledge can be necessary. It draws on a number of philosophical concepts such as necessity, the

causal theory of reference A causal theory of reference or historical chain theory of reference is a theory of how terms acquire specific referents based on evidence. Such theories have been used to describe many referring terms, particularly logical terms, proper names, and ...

, rigidity, and the a priori a posteriori distinction. It was first introduced by philosopher

Saul Kripke Saul Aaron Kripke (; November 13, 1940 – September 15, 2022) was an American philosopher and logician in the analytic tradition. He was a Distinguished Professor of Philosophy at the Graduate Center of the City University of New York and emerit ...

in his 1970 series of lectures at Princeton University. The transcript of these lectures was then compiled and assembled into his seminal book, ''

Naming and Necessity ''Naming and Necessity'' is a 1980 book with the transcript of three lectures, given by the philosopher Saul Kripke, at Princeton University in 1970, in which he dealt with the debates of proper names in the philosophy of language. The transcript ...

''.

Main argument for a posteriori necessity

Here is an overview of the argument: :(P1) 'Hesperus' is a proper name that refers to the evening star. 'Phosphorus' is also a proper name and it refers to the morning star. But the evening star and the morning star are the same planetary body (Venus). So both names designate Venus. :(P2) If both names designate rigidly, they designate the same object (Venus) in every possible world. Therefore (by the definition of 'necessary') 'Hesperus = Phosphorus' is necessarily true. If it is the case that in all possible worlds the identity claim “Hesperus is Phosphorus” is true, the statement is necessary. :(P3) The fact that Hesperus is Phosphorus was discovered by empirical observation, and it could not have been discovered a priori. Therefore, it is known a posteriori. :(C) Therefore, it is possible for knowledge obtained a posteriori to be necessary. Other instances of a posteriori necessary truths include: "H2O is water".

Significance

''Naming and Necessity'' is among the most important philosophical works of the 20th century. A posteriori necessity existing would make the distinction between a prioricity, analyticity, and necessity harder to discern because they were previously thought to be largely separated from the a posteriori, the synthetic, and the contingent. (a) ''P'' is a priori iff ''P'' is necessary. (b) ''P'' is a posteriori iff ''P'' is

contingent Contingency or Contingent may refer to: * Contingency (philosophy), in philosophy and logic * Contingency plan, in planning * Contingency table, in statistics * Contingency theory, in organizational theory * Contingency theory (biology) in evoluti ...

Hilary Putnam Hilary Whitehall Putnam (; July 31, 1926 – March 13, 2016) was an American philosopher, mathematician, and computer scientist, and a major figure in analytic philosophy in the second half of the 20th century. He made significant contributions ...

comments on the significance of Kripke’s counter-examples, "Since Kant there has been a big split between philosophers who thought that all necessary truths were analytic and philosophers who thought that some necessary truths were synthetic a priori. But none of these philosophers thought that a (metaphysically) necessary truth could fail to be a priori.”

References

{{Reflist Epistemology

Main argument for a posteriori necessity

Significance

See also

References