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Obituary

Saul Kripke (13th November 1940 - 15th September 2022)

Stefan Rinner tells us about Kripke’s theories and why they matter.

As philosophers, we find again and again that ‘What is philosophy?’ is as difficult a question as any other philosophical problem. However, in the middle of the twentieth century, there seemed to be a simple answer to it: philosophers primarily analyze concepts. Accordingly, answering the question “What is knowledge?”, for example, was tantamount to answering the question “What is the meaning of the term ‘knowledge’?”

The reason why philosophers thought that their craft consisted primarily in analyzing concepts was a certain theory of meaning – the so-called ‘description theory of meaning’. According to this, the meaning of a referring expression is given by a description uniquely picking out the referent of the term, i.e. the thing in the world to which the term refers. For example a proper name such as ‘Napoleon’ has the same meaning as a description such as “the French emperor who was defeated at Waterloo”, or as a bundle of such descriptions. Thus, answering the question of who Napoleon was is just giving the meaning of the name in terms of a description: “Napoleon was the French emperor who lost at Waterloo.” Similarly, answering the question “What is knowledge?” is just giving the meaning of the term ‘knowledge’ through an equivalent description. In this way the description theory of meaning gave a simple understanding of what philosophical problems and their solutions are. At only thirty years of age, in 1970, Saul Kripke shattered this understanding of philosophy with a groundbreaking lecture series which later became his book Naming and Necessity (1980).

Saul Kripke by Darren Mcandrew

In these lectures Kripke put forward powerful arguments against the description theory of meaning, in particular against the description theory of proper names. According to that hypothesis, the referent of a name is determined by a description, or by a bundle of descriptions, that the speaker associates with the name. Against this, Kripke points out that there are cases where the descriptions that a speaker associates with a name do not pick out its actual referent. For instance, many speakers who know the name at all associate the name ‘Peano’ only with the description “the discoverer of the Peano Axioms in mathematics.” But that description actually picks out Richard Dedekind, not Giuseppe Peano! Nevertheless, as Kripke points out, with their uses of ‘Peano’ the speakers are referring to Peano. Furthermore, there are cases where speakers do not even associate a name with a description, in the sense that they don’t pick out exactly one object with their description, but nevertheless, that is how they use the name. For example, with the name ‘Cicero’ many speakers only associate the property of ‘being a Roman orator’.

Therefore, Kripke proposes that rather than being determined by a description or set of descriptions, the referent of a name is determined by a so-called ‘communicational chain’. The first link of such a chain is someone dubbing an object with its name. The other links of the chain are borrowings of this name from one speaker to the next. This is also known as ‘the causal theory of reference’.

Kripke goes on in Naming and Necessity to extend the causal theory of reference from proper names to natural kind terms, such as ‘water’ and ‘tiger’. A similar theory had been proposed by Hilary Putnam, and, following the work of Kripke and Putnam, philosophers such as Tyler Burge and Michael Devitt have argued for different extensions of the causal theory of reference beyond proper names and natural kind terms, covering, among other things, social and artificial kind terms. In this way, Kripke’s work has completely changed professional philosophers’ understanding of meaning, from an internalist view – according to which the meanings of our expressions are descriptions or bundles of descriptions in our heads – to an externalist view – which claims that the meanings of our expressions are nothing other than their referents out there in the world. This is why causal theories of reference are often also referred to as ‘semantic externalism’.

Semantic externalism has had far-reaching implications for philosophy. For example, by arguing for a causal theory of reference for proper names and natural kind terms, Kripke single-handedly rehabilitated a philosophical discipline that had suffered badly under the description theory of meaning – metaphysics. After all, if questions regarding the nature of things such as knowledge or Napoleon are only questions regarding the meanings of the linguistic expressions ‘knowledge’ and ‘Napoleon’, then this implies that when talking about reality we are not in fact studying reality itself, but only human language use. In which case metaphysics, as the study of the fundamental nature of reality, seems to have no real subject matter.

Playing into the hands of the description theory and the associated linguistic conception of metaphysics, was the fact that, before Kripke, many philosophers thought that everything that is necessarily true can be known by reasoning alone – that is, a priori – and that everything that could be known a priori was necessarily true. From this they concluded that everything that is necessarily true has to be true because of the meaning of the sentences used to formulate those truths. This would explain why necessary truths can be known by reasoning alone.

Kripke countered this line of thought with one of his main insights, which is that it does not follow from the fact that a sentence is necessarily true, that its truth can be known by reasoning alone. For instance, if Goldbach’s conjecture (a theorem about prime numbers) is true, then it is necessarily true: it can never have been false. However, as Kripke points out, from this it follows neither that the truth of Goldbach’s conjecture can be known by reasoning alone, nor, indeed, that it can be known at all.

Once Kripke had rejected the description theory of meaning and the equating of necessity with being a priori, the way was paved for his intuitive view that both objects and natural kinds have properties that are necessary for their particular existence. This view is known as essentialism. For instance, according to Kripke, Napoleon could not have existed without being a human being: if something wasn’t human, it wasn’t Napoleon. Similarly, Kripke argues that tigers could not have existed without being mammals. Hence, even though it cannot be known by reasoning alone that ‘Tigers are mammals’ is true, nevertheless, for Kripke, the sentence is necessarily true, and moreover, it tells us something about the nature of tigers independent of the meaning (that is, definition) of the natural kind term ‘tiger’.

Rehabilitating metaphysics in this way was also of great importance for Kripke’s work on modal logic. This is the brand of logic involving modal operators, such as ‘It is necessary that…’ or by contrast ‘It is possible that…’. In particular it was important in connection with quantified modal logic, which studies the interplay of the modal operators of ‘possible’ and ‘necessary’ with logical quantifiers such as ‘every’ and ‘some’. While he was still in his teens, Kripke developed an original interpretation of quantified modal logic using so-called ‘possible worlds’.

Nevertheless, many philosophers were skeptical when it came to quantified modal logic, since by allowing quantification (some, every, etc) into modal contexts (for example ‘Every tiger is such that it is necessary that it is a mammal’), quantified modal logic seems to presuppose some form of essentialism. Therefore, by rehabilitating essentialism, Kripke also vindicated his early work on quantified modal logic.

Kripke was sometimes described as having a certain fondness for philosophical puzzles and paradoxes, and this is certainly true. However, as has been suggested in these few lines, his views are probably as comprehensive as they can be for a philosopher of the twentieth century, replacing the linguistic conception of philosophy which was predominant in the middle of the twentieth century with a view that puts metaphysical considerations first. In this way, Kripke leaves us not only with a better understanding of philosophy, but also with a better appreciation of his and our fondness for its puzzles and paradoxes.

Saul Kripke died on September 15 2022 at the age of 81.

Stefan Rinner is an interim professor in philosophy of language at the University of Hamburg.