Hume's Fork (A-Level Philosophy)
While known primarily as a historian during his lifetime, David Hume (1711-1776) has become a central figure in the history of Western philosophy. As Hegel notes in his Lectures on the History of Philosophy, "Hume is important… because he carried the principles of empiricism to their logical conclusion, leading to scepticism. This scepticism, by destroying false notions of knowledge, prepared the way for Kant."
Indeed, Hume famously woke Kant from his “dogmatic slumber” through his sceptical arguments against fundamental concepts like necessity and causation. Underpinning this challenge is a notion that is indispensable for A-level Philosophy students: Hume’s Fork. This epistemological tool proposes a rigid division between two distinct types of knowledge: Matters of Fact and Relations of Ideas. Anything which falls outside of these two categories (including cosmological and teleological arguments) should, in Hume’s words, be committed “to the flames”.
In this blog, expert tutors critically examine Hume's Fork, outlining its fundamental concepts and addressing key objections. We'll then explore its significant influence on subsequent philosophers, notably Kant and the Logical Positivists, and demonstrate its direct relevance to various sections of your A-Level Philosophy exams. The article will also be of use to A-level Religious Studies students, as Hume’s Fork provides a powerful critique of attempts to prove God’s existence.
What is Hume's Fork?
In his groundbreaking work, An Enquiry Concerning Human Understanding (1748), Hume famously states that:
"If we take in our hand any volume; of divinity or school metaphysics, for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the flames: for it can contain nothing but sophistry and illusion" (Section XII, Part III).
Such a radical rejection of traditional metaphysics and theology is underpinned by Hume’s Fork. This strict dichotomy proposes that all objects of human reason or inquiry can be neatly divided into two, mutually exclusive categories:
1. Relations of Ideas
These are propositions discoverable purely by reason and thought, without any need for external experience of the world. Their truth is independent of anything actually existing.
Analytic: The predicate (what is said about the subject) is already contained within the subject itself (e.g., "All bachelors are unmarried men").
A priori: Their truth can be known prior to or independently of any sensory experience.
Necessarily True: Their negation (saying the opposite) leads to a logical contradiction; they simply cannot be false.
Demonstrable: Their truth can be logically proven or demonstrated through a series of deductive steps.
As Hume states: "Propositions of this kind are discoverable by the mere operation of thought, without dependence on what is anywhere existent in the universe." (Section IV, Part 1). These "Relations of Ideas" are crucially understood as necessary truths: they must be true and cannot possibly be otherwise. Their truth is independent of any actual state of affairs in the world; denying them would result in a logical contradiction. Examples include mathematical statements (like 2+2=4), the foundational laws of logic (e.g., all triangles, by definition, must have three sides), and any truth derived purely from the definition of terms.
2. Matters of Fact
These are propositions whose truth depends entirely on experience and observation of the world. Their truth is contingent on how reality happens to be.
Synthetic: The predicate adds new information to the subject; it's not already contained within it (e.g., "The cat is on the mat").
A posteriori: Their truth can only be known after or through sensory experience.
Contingent: Their negation implies no contradiction; the opposite is perfectly possible and conceivable (e.g., "The sun might not rise tomorrow").
Not Demonstrable: Their truth cannot be logically proven; they can only be confirmed or denied by observing reality.
Thus, for Hume, Matters of Fact are characterised by their contingency. This means that their truth is dependent on how the world happens to be, and its opposite is always logically possible, no matter how unlikely it seems based on our experience. Hume emphasises this by stating: "The contrary of every matter of fact is still possible; because it can never imply a contradiction, and is conceived by the mind with the same facility and distinctness, as if ever so conformable to reality. That the sun will not rise tomorrow is no less intelligible a proposition, and implies no more contradiction, than the affirmation, that it will rise." (Section IV, Part 1)
This means that while we might strongly expect the sun to rise based on past observations, we can still conceive of a world where it doesn't, without any logical absurdity. There's no inherent logical necessity that forces the sun to rise. This contrasts sharply with a "Relation of Ideas," where denying its truth leads to a contradiction (e.g., you cannot conceive of a square circle). Therefore, all knowledge derived from observation, such as scientific findings, historical events, or simple empirical claims like "the mug is on the table," belongs to this contingent category, as their truth is not guaranteed by logic alone but by what we experience.
Is the Fork Self-refuting?
A significant criticism often raised against Hume's Fork questions whether the Fork itself, as a statement, fits into either of its own categories. The statement "All truths are either relations of ideas or matters of fact" presents a dilemma:
If this statement is a relation of ideas, it would be a tautology – true by definition, but telling us nothing new or substantial about how knowledge works in the real world. This would make it seem arbitrary.
If it's a matter of fact, then its opposite must be conceivable without contradiction (meaning there could be a third type of truth). If so, the Fork's claim to be a universal and exhaustive division of all knowledge is undermined, making it less foundational than intended.
This challenge suggests the Fork might be self-refuting, similar to the objection that A.J. Ayer's Verification Principle (which claims only analytic or empirically verifiable statements are meaningful) cannot itself be verified by its own criteria.
Hume would likely respond that the Fork is a methodological assertion or an empirical observation about the limits of human understanding itself. If pressed to categorise it, he might admit it functions as a very high-level "Matter of Fact" – a sweeping generalisation derived from his rigorous analysis of all human thought and experience. While the negation of a matter of fact is conceivable, this doesn't automatically render the original statement false or useless. For Hume, the Fork is profoundly useful because it clearly exposes areas of inquiry that lack any real basis in either reason or experience.
He would then shift the burden of proof back to the critic: "If you believe there is a third category of truth, then demonstrate it! Provide a proposition that is genuinely meaningful, informative, yet neither discoverable by pure thought alone nor verifiable by experience." Since, in Hume's view, no philosopher has ever successfully done this, the Fork remains, for him, a valid and effective framework for evaluating knowledge, despite this meta-level objection. In the next section, we will examine a philosopher who took Hume up on this challenge by striving to demonstrate an essential third category of knowledge.
Kant's Transcendental Idealism
Kant read Hume and was, in his own words, "awakened from his dogmatic slumber". Hume’s radical sceptical challenge to traditional metaphysical concepts, particularly regarding necessary existence and causation, dislodged Kant from the dominant Leibnizian-Wolfian rationalist philosophy of his era and inspired him to critically probe the limits of pure reason.
Kant largely agrees with Hume's distinction between analytic/synthetic and a priori/a posteriori but believed Hume had missed a crucial third category: synthetic a priori judgments.
In the Critique of Pure Reason, Kant argued that while all knowledge begins with experience, it doesn't follow that it all arises out of experience. He proposed that the mind is not a blank slate (as empiricists like Hume suggested) but comes equipped with innate structures – categories of understanding (e.g., causality, substance, unity) and forms of intuition (space and time). These structures are a priori because they are built into the very way our minds process experience, not derived from it. Therefore, for Kant, certain judgments about the world are:
Synthetic: They expand our knowledge; the predicate is not contained in the subject.
A priori: They are universal and necessary, not derived from particular experiences.
Examples of Kant's Synthetic A Priori Judgments:
Mathematics: Kant argued that "7 + 5 = 12" is synthetic because the concept of "12" is not contained within "7 + 5"; it requires an act of intuition or construction. Yet, it's a priori because its truth is universal and necessary, not contingent on observation.
Causation: Directly addressing Hume, Kant argued that "Every event has a cause" is a synthetic a priori truth. We don't derive causality from experience; rather, our minds impose the concept of causality on our experiences to make sense of them. It's a fundamental way our understanding structures the phenomenal world.
Moral Laws: Kant's ethical philosophy also relied on synthetic a priori principles, such as the Categorical Imperative, which he believed was universally prescribed by reason regardless of empirical consequences or desire-driven inclinations.
Kant essentially argues that Hume's Fork is incomplete. While it correctly identifies two types of judgment, it overlooks the possibility of necessary and universal truths that nevertheless provide new information about the world, made possible by the active structuring role of the human mind. For A-level Philosophy students, Kant's philosophy can be understood as an attempt to integrate the strengths of both rationalism and empiricism, offering a path forward from Hume's profound scepticism.
Influence on Logical Positivism
Nevertheless, not all post-Kantain philosophers reject Hume’s Fork. The idea exerted a potent influence on the Logical Positivists, particularly A.J. Ayer. In his famous work Language, Truth, and Logic, Ayer adopts a modern version of Hume's Fork through his Verification Principle. This principle asserts that a statement is only meaningful if it is either analytic (a "relation of ideas," true by definition) or empirically verifiable (a "matter of fact," confirmable by experience). Any statement failing this test, such as those in metaphysics, ethics, or theology, is deemed literally meaningless, echoing Hume's own dismissive conclusion that metaphysics should be “committed to the flames”. Ayer's work thus shows how Hume's sharp distinction may be used to sweep away vast areas of traditional philosophical inquiry, including the metaphysics of God.
Nevertheless, paralleling the above objection to Hume’s Fork, Ayer's Principle is famously self-refuting. It is not a mere definition or logical tautology, nor can its truth be confirmed through empirical observation or scientific experiment. Therefore, by its own criteria, the Verification Principle renders itself meaningless, undermining its very foundation and its ability to dismiss other philosophical or theological claims as meaningless.
Application to A-Level Philosophy
Hume's Fork is a powerful analytical tool with broad applicability across the A-level Philosophy syllabus. Its challenge to claims of synthetic a priori knowledge can potentially expose vulnerabilities in any theory that posits causal connections beyond direct empirical observation. Thus, in the Philosophy of Mind, substance dualism (which infers an unobservable causal link between a non-physical mind and a physical body) is particularly susceptible to Hume’s critique. Nevertheless, the Fork's most direct impact on the syllabus is seen in two key areas:
1. Epistemology:
Limits of Knowledge: It sets strict boundaries on what can be genuinely known. Anything that doesn't fit either "Relations of Ideas" (analytic, necessary, a priori) or "Matters of Fact" (synthetic, contingent, a posteriori) is deemed meaningless, fictitious, or based on mere custom.
Challenge to Rationalism: Hume's assertion that all synthetic knowledge is a posteriori directly challenges rationalists like Descartes. In his Meditations, Descartes famously sought a priori certainty, moving from "I think, therefore I am" to arguments for God's existence based on the innate idea of a perfect being (Meditation III). Hume's Fork fundamentally refutes this: by insisting new knowledge must come from experience, it renders Descartes' attempts to deduce truths about God or the world from pure reason and innate ideas as lacking empirical verification, thus falling outside the realm of genuine knowledge. Similarly, Hume's Fork directly challenges Leibniz's Principle of Sufficient Reason (PSR). Leibniz asserted that everything must have a sufficient reason for its being. Hume, however, would ask if the PSR is a "Relation of Ideas" or a "Matter of Fact." He'd argue it's not a Relation of Ideas because its negation isn't a contradiction, and it's not a Matter of Fact because we don't experience "sufficient reason" or "necessary connection." Thus, since the PSR fits neither category, Hume dismisses it as a product of custom or a fictitious idea, incapable of providing true knowledge.
2. Metaphysics:
Critique of Causation: This is perhaps its most famous application. Hume uses the Fork to argue that our belief in necessary causal connections is not justified. The statement "every event has a cause" is neither a relation of ideas (its negation is conceivable) nor a matter of fact (we only observe conjunctions, not necessary connections). This fundamentally challenges metaphysical claims about the universe having inherent causal structures, exemplified by the Kalam Argument.
Natural Theology: Hume's Fork provides a powerful critique of traditional arguments for God's existence. As we have seen, Hume argues that our belief in causation is based merely on observed "constant conjunctions," not a necessary connection. This means we cannot logically infer an unobserved cause from an observed effect. This directly challenges Cosmological Arguments, such as those by Aquinas. If causation is not a necessary link, then the chain of causes leading to a First Cause (God) is undermined, lacking rational necessity. Similarly, it impacts Teleological Arguments (from design), like William Paley's Watchmaker analogy. In Dialogues Concerning Natural Religion (1779), Hume argues we have no experience of universe-making; therefore, we cannot logically infer a divine designer for the universe in the same way we infer a watchmaker for a watch. The Fork limits our ability to reason from observed phenomena to metaphysical conclusions about God.
Nature of the Self: Hume's empiricism, flowing from the Fork's principles, led him to a radical conclusion about the self: it is merely a "bundle of perceptions," with no underlying, enduring substance. This directly contradicts John Locke's conception of personal identity. Locke argues the self is a continuous entity unified by consciousness and memory extending back through time. However, Hume's rigorous empiricism finds no "impression" of this supposed enduring self; only fleeting perceptions of sensations and thoughts. Thus, for Hume, the self is simply "a bundle or collection of different perceptions, which succeed each other with inconceivable rapidity, and are in a perpetual flux and movement," entirely dismantling Locke's notion of a substantial, unified "I".
Conclusion
Hume's Fork presents a radical epistemological challenge, forcing us to re-evaluate the limits of knowledge and the true task of philosophical inquiry. As Arthur Schopenhauer observes in The World as Will and Representation, Vol. 1, Hume "exposed with the most extreme clarity the fact that we can never know the ultimate ground or inner nature of things from mere perception, because this can only reveal particular phenomena and not the connection between them." This profound insight leaves a stark choice: either accept this inherent restriction on knowledge, or seek a new philosophical path that could account for connections beyond mere observation. The latter task, of finding a third category of knowledge beyond Hume’s strict dichotomy, became the driving force behind Kant's transcendental idealism, which inspired philosophers such as Hegel.
For A-Level Philosophy exams, it's essential to understand how (if valid) Hume’s Fork fatally undermines traditional metaphysical claims, particularly regarding causation and arguments for God's existence. It can act as a potent objection to cosmological and teleological arguments by challenging their fundamental premises, such as the Principle of Sufficient Reason. In Epistemology, the Fork provides a formidable challenge to the Rationalism of Descartes and Leibniz by asserting that only empirical experience can yield new knowledge. However, you may effectively counter the Fork by arguing that it is self-refuting, or by demonstrating (as Kant famously maintains) that it is not exhaustive and overlooks a vital third category of knowledge: the synthetic a priori.
