Saul Kripke: Philosophy, Quotes & Legacy

Biography

Saul Aaron Kripke was born on November 13, 1940, in Bay Shore, New York, and grew up in Omaha, Nebraska, the eldest of three children in a Jewish family. His father, Dorothy Kripke's husband Rabbi Myer Kripke, led a Conservative congregation in Omaha, and his mother, Dorothy Kripke, was an author of Jewish educational materials. The Kripke household was intellectually rigorous — the family dinner table was a place where theological, mathematical, and philosophical questions were discussed with seriousness — and young Saul displayed an astonishing precocity that would become legendary in philosophical circles. By the age of six, he had taught himself to read Hebrew. By elementary school, he was working through algebra and geometry on his own. And by the time he was a teenager, he had plunged into the most advanced literature on mathematical logic, reading the works of Wittgenstein, Bertrand Russell, and the founders of modern modal logic with an understanding that astonished his teachers and mentors.

The story of Kripke's early brilliance has become part of the folklore of analytic philosophy. At the age of fourteen, he wrote a letter to the logician Haskell Curry pointing out an error in a published proof. At sixteen, he completed his first original paper in modal logic, a formalization of the semantics for modal systems that would eventually bear his name. The paper, published in the Journal of Symbolic Logic in 1959 when Kripke was just eighteen, introduced what came to be known as Kripke semantics — a possible-worlds framework for interpreting modal operators like "necessarily" and "possibly" that revolutionized the field and remains the standard approach to this day. Kripke had, in effect, solved one of the central problems in mathematical logic while still in high school, and the philosophical community took notice.

Kripke entered Harvard University in 1958, where he studied mathematics and philosophy. He was already a recognized figure in logic by the time he arrived, and his fellow students and professors quickly realized that they were in the presence of an extraordinary talent. Among his classmates was Hilary Putnam, who would later become both a colleague and a philosophical interlocutor. Kripke graduated summa cum laude in 1962 with a bachelor's degree in mathematics, having written a senior thesis on modal logic. He was elected to Phi Beta Kappa and received Harvard's prestigious Bowdoin Prize. Despite the obvious expectation that he would pursue a doctorate, Kripke never completed a PhD — a fact that did not prevent him from becoming one of the most sought-after philosophers in the world. His published work, beginning with the 1959 paper and continuing through a series of landmark articles in the 1960s, was more than sufficient to establish his credentials, and academic institutions competed for his services.

After graduation, Kripke remained at Harvard as a member of the Society of Fellows from 1962 to 1968, a position that gave him the freedom to pursue his research without the burden of conventional academic duties. In 1968, he moved to Rockefeller University in New York, where he taught until 1976. He then accepted a position at Princeton University as McCosh Professor of Philosophy, one of the most prestigious chairs in American philosophy. Princeton in the late 1970s and 1980s was a center of analytic philosophy, and Kripke's presence — along with that of figures like Thomas Nagel, Harry Frankfurt, and Gilbert Harman — made it one of the most dynamic philosophy departments in the world. In 1998, Kripke moved to the Graduate Center of the City University of New York (CUNY), where he held a Distinguished Professorship. He continued to write and lecture, though his publication rate slowed considerably in his later years — a source of frustration for his admirers, who knew that many of his most important ideas were circulating in unpublished lectures and manuscripts.

Kripke has been described by colleagues as shy, intensely private, and utterly uninterested in the social rituals of academic life. He rarely gave interviews, declined most invitations to contribute to collected volumes, and preferred to work out his ideas in lectures and seminars rather than in published papers. The Stanford Encyclopedia of Philosophy notes that much of Kripke's most influential work was originally delivered orally — in lectures, conference talks, and seminar discussions — and that the published record captures only a fraction of his philosophical output. This oral tradition, combined with the extraordinary depth and originality of the ideas he did publish, has given Kripke a unique status in contemporary philosophy: he is both one of the most influential philosophers of the last century and one of the most enigmatic.

Philosophy

Kripke's philosophical work spans modal logic, the philosophy of language, metaphysics, epistemology, and the philosophy of mind, and it is unified by a set of methodological commitments that distinguish it from the logical positivism and ordinary language philosophy that dominated mid-century analytic philosophy. Kripke takes metaphysical questions seriously — questions about what is necessary, what is possible, what is essential to a thing — and he uses the tools of formal logic to clarify and answer them. He is skeptical of attempts to deflate metaphysical claims into linguistic or epistemological ones, and he is willing to follow the logic of modal reasoning wherever it leads, even when the results challenge deeply entrenched philosophical orthodoxies.

Kripke Semantics and Modal Logic

Kripke's earliest and most technically influential contribution is the possible-worlds semantics for modal logic, now universally known as Kripke semantics. Before Kripke, modal logic suffered from a foundational problem: the operators "necessarily" and "possibly" were introduced syntactically, by adding axioms to propositional logic, but there was no clear semantic interpretation that explained what these operators meant or why one set of axioms was preferable to another. Kripke solved this problem by introducing the notion of a "frame" — a set of "possible worlds" connected by an "accessibility relation" that determines which worlds are accessible from which others. A statement is necessarily true at a world if it is true at all worlds accessible from that world, and possibly true if it is true at at least one accessible world. Different modal systems (T, S4, S5, etc.) correspond to different constraints on the accessibility relation (reflexivity, transitivity, symmetry), and these constraints can be given natural and intuitive interpretations.

This framework, which Kripke developed in a series of papers from 1959 to 1963, transformed modal logic from a curious formal exercise into a powerful and widely applicable tool. It provided the semantic foundations for not only modal logic but also intuitionistic logic, tense logic, deontic logic, and conditional logic. And it made the concept of "possible worlds" — which had been a somewhat mysterious notion in the work of Leibniz — into a rigorous and philosophically fruitful tool. The possible-worlds framework would become central to Kripke's later work on reference and necessity, and it has become a standard part of the philosopher's toolkit across metaphysics, epistemology, and the philosophy of language.

Rigid Designation

Kripke's most famous philosophical contribution is the theory of rigid designation, developed in the three lectures delivered at Princeton in January 1970 and published as Naming and Necessity in 1972 (and as a standalone book in 1980). The theory addresses a question that had been at the center of the philosophy of language since Gottlob Frege: how do proper names refer to the objects they denote? Frege and Russell had argued that a name refers to its bearer by means of a description — "Aristotle" means "the pupil of Plato who taught Alexander the Great," for example. On this descriptivist view, the reference of a name is mediated by the descriptions that speakers associate with it.

Kripke argued that this picture is fundamentally wrong. A proper name, he claimed, is a rigid designator: it refers to the same object in every possible world in which that object exists. The name "Aristotle" refers to Aristotle not only in the actual world but in every possible world in which Aristotle exists — including worlds where Aristotle never taught Alexander, never studied with Plato, and never did any of the things we ordinarily associate with him. If Aristotle had become a fisherman instead of a philosopher, he would still be Aristotle, and the name "Aristotle" would still refer to him. This means that the reference of a name cannot be fixed by the descriptions speakers associate with it, because those descriptions could be false of the bearer in other possible worlds, and yet the name would still refer to that same person.

Kripke replaced the descriptivist picture with what he called the causal-historical theory of reference. On this view, a name refers to its bearer not because of any description the speaker associates with it but because of a causal chain that links the speaker's use of the name back to the original "baptism" in which the object was first named. When I use the name "Aristotle," I refer to the ancient philosopher not because I know a set of descriptions that uniquely identify him, but because my use of the name is part of a causal chain of communication that traces back through my teachers, through books, through the transmission of the name across centuries, to the original naming of that particular individual. The reference is fixed by the causal history of the name's use, not by the speaker's descriptive knowledge.

Necessary A Posteriori Truths

The theory of rigid designation led Kripke to one of his most philosophically revolutionary claims: the existence of necessary a posteriori truths. Ever since Kant, philosophers had assumed that necessity and apriority go hand in hand — that what is necessary can be known a priori, and what is known a priori is necessary. Kripke shattered this assumption by showing that some truths are both necessary and knowable only through empirical investigation.

The paradigmatic example is the identity statement "Hesperus is Phosphorus," or equivalently, "the Morning Star is the Evening Star." Hesperus and Phosphorus are both names for the planet Venus, but this was not always known. Ancient astronomers believed they were two different celestial bodies, and it took empirical observation to discover that they are the same. The identity "Hesperus is Phosphorus" is therefore a posteriori — it required experience to establish. But since both "Hesperus" and "Phosphorus" are rigid designators, they refer to the same object (Venus) in every possible world. If Hesperus is Phosphorus, then they are necessarily identical — there is no possible world in which Venus is not identical to itself. The statement "Hesperus is Phosphorus" is therefore both necessary and a posteriori, a combination that the Kantian tradition had declared impossible.

This result has profound implications. It means that metaphysical necessity — what must be the case — is not the same as epistemological apriority — what can be known without experience. Some things that are metaphysically necessary can only be discovered empirically, and some things that can be known a priori are not metaphysically necessary. Kripke also introduced the complementary notion of the contingent a priori: statements that can be known without experience but that are not metaphysically necessary, such as "the standard meter stick is one meter long" (assuming the stick is used to define the meter). Together, these distinctions dissolved a framework that had structured epistemology since Kant and opened up new territory for metaphysical inquiry.

The Causal Theory of Reference and Natural Kinds

Kripke extended his theory of proper names to natural kind terms — terms like "water," "gold," "tiger," and "heat" that pick out natural categories rather than individual objects. He argued, following Hilary Putnam's complementary work on semantic externalism, that natural kind terms are also rigid designators: "water" refers to H2O in every possible world, not just in the actual world. This means that it is a necessary truth that water is H2O — if a substance is not H2O, it is not water, even if it looks and behaves exactly like water. Kripke acknowledged that this is an empirical discovery, not an a priori truth, which makes it another example of a necessary a posteriori truth.

This position has striking consequences for the philosophy of mind. If "pain" is a rigid designator for a particular kind of sensation, and if that sensation turns out to be identical to C-fiber firing (as some identity theorists claimed), then the identity "pain is C-fiber firing" would be necessary. But it would also be a posteriori, since we had to discover the neural correlate of pain through empirical research. Kripke noted, however, that there seems to be an important disanalogy between "pain is C-fiber firing" and "water is H2O." In the case of water, we can imagine a substance that looks and behaves like water but is not H2O — and we would say it is not water. But in the case of pain, we cannot coherently imagine a situation in which C-fiber firing occurs but there is no pain — the feeling of pain seems essential to it in a way that the chemical structure of water is not essential to its superficial properties. This argument, brief but powerful, has been taken by many philosophers as a serious challenge to physicalist theories of mind.

Kripkenstein: The Rule-Following Paradox

In Wittgenstein on Rules and Private Language (1982), Kripke offered an interpretation of Wittgenstein's rule-following considerations in Philosophical Investigations that became known as "Kripkenstein." The central puzzle is this: when we follow a rule — say, the rule for addition — what determines that we are following that rule rather than some other rule that happens to agree with it in all the cases we have considered so far? If I have never computed 68 + 57 before, what makes it true that the correct answer is 125 rather than 5 (which would be the answer if "plus" meant "quus," a function that agrees with addition for all numbers below 68 but gives 5 for 68 + 57)? There seem to be infinitely many rules compatible with my past behavior, and no fact about my mental state — no intention, no mental image, no disposition — that uniquely determines which rule I am following.

Kripke's interpretation, whether or not it faithfully represents Wittgenstein's own views, has generated an enormous literature. The skeptical conclusion — that there is no fact of the matter about what rule I am following — seems unacceptable, and Kripke offered a "skeptical solution" based on the idea that rule-following is a communal practice: what makes my use of "plus" correct is not that it corresponds to some abstract rule in my head, but that it agrees with the practices of my linguistic community. The rule-following paradox raises deep questions about meaning, normativity, and the nature of mathematical truth, and it remains one of the most discussed arguments in contemporary philosophy.

Key Ideas

The Essentiality of Origin

Kripke argued that certain properties of an object are essential to it — the object could not exist without them. In particular, he suggested that the origin of an object is essential: the table made from a particular piece of wood could not have been made from a different piece of wood. It would be a different table, not the same table made from different material. This view, which connects the modal logic of identity to metaphysical questions about essence and accident, has been influential in contemporary metaphysics and has generated extensive debate about the criteria of identity and the nature of essential properties.

Against the Identity Theory of Mind

Kripke's modal argument against the identity theory of mind is one of the most influential arguments in the philosophy of mind. If pain is identical to C-fiber firing, then the identity is necessary (since identities between rigid designators are necessary). But we can coherently imagine C-fiber firing occurring without pain — or at least, the conceivability of C-fiber firing without pain seems more intelligible than the conceivability of water without H2O. This asymmetry, Kripke suggested, means that the identity theory is in trouble: if pain and C-fiber firing were truly identical, it should be as inconceivable to have one without the other as it is to have water without H2O. The argument does not prove that physicalism is false, but it identifies a pressure point that physicalists must address.

Famous Quotes

Kripke's prose, when he chooses to publish it, is precise, direct, and free of unnecessary ornament. His formulations have become touchstones of contemporary philosophy:

"Let's call something a rigid designator if in every possible world it designates the same object, a nonrigid or accidental designator if that is not the case."

This definition, from the Preface to Naming and Necessity (1980), introduced the concept that would reshape the philosophy of language and metaphysics. It is the cornerstone of Kripke's challenge to the descriptivist tradition that ran from Frege through Russell to W.V.O. Quine.

"If Hesperus is Phosphorus, then Hesperus is necessarily Phosphorus."

This deceptively simple statement, from Lecture II of Naming and Necessity, encapsulates the argument for necessary a posteriori truths. If two rigid designators name the same object, their identity is necessary — even though discovering that identity may require empirical investigation.

"One of the great fallacies of positivism has been to think that necessary truths can only be analytic and a priori."

This observation, from the discussion period following the Naming and Necessity lectures, captures the broader philosophical significance of Kripke's work: the dismantling of the Kantian framework that had structured epistemology and the philosophy of language for two centuries.

Legacy

Saul Kripke's impact on analytic philosophy has been compared to that of Wittgenstein in the earlier twentieth century: a single thinker whose ideas fundamentally reconfigured the landscape and set the agenda for generations of subsequent work. The Stanford Encyclopedia of Philosophy describes Naming and Necessity as one of the most influential philosophical works of the twentieth century, a judgment that few would dispute. The theory of rigid designation overturned the descriptivist orthodoxy that had dominated the philosophy of language since Frege, the concept of necessary a posteriori truths dissolved the Kantian synthesis of necessity and apriority, and the causal theory of reference provided a new framework for understanding how language connects to the world.

Kripke's influence extends far beyond the philosophy of language. His work on modal logic provided the semantic foundations that made possible the explosive growth of modal metaphysics in the late twentieth century — the flourishing of work on possible worlds, essential properties, counterfactuals, and the logic of causation. His arguments about the identity theory of mind have been central to debates about consciousness and physicalism, and his interpretation of Wittgenstein's rule-following paradox has generated a literature that shows no signs of abating. In metaphysics, his suggestions about the essentiality of origin and the nature of identity have shaped debates about personal identity, persistence, and constitution.

What makes Kripke's legacy distinctive is the peculiar combination of technical rigor and philosophical vision that characterizes his work. He is a mathematician and logician of the first rank — his semantics for modal logic is a formal achievement of the highest order — but he uses formal tools not as ends in themselves but as instruments for addressing deep philosophical questions about necessity, reference, and the nature of reality. His work demonstrates that the technical resources of analytic philosophy, far from being a sterile exercise in formalism, can illuminate questions that have animated philosophy since Aristotle: What is necessary? What is possible? What makes it the case that our words refer to the things they do? That these questions can be asked with precision and answered with insight, without sacrificing depth for clarity or clarity for depth, is the enduring lesson of Kripke's philosophical career.