Haecceitism

Origins and Core Idea

Haecceitism is a metaphysical doctrine about individuality and identity. It is historically rooted in the medieval notion of haecceitas—Latin for “thisness”—most closely associated with John Duns Scotus. Scotus introduced haecceitas as a non-qualitative principle of individuation: what makes this particular human numerically distinct from any qualitatively indistinguishable human, or this particular lump of matter distinct from another just like it.

Modern haecceitism is not simply the acceptance of haecceitas as Scotus conceived it, but a more general stance: that there are primitive facts about which individual is which, over and above all qualitative facts (facts about properties, relations, and structures). On this view, two situations or possible worlds can be qualitatively identical while differing in “who is who” or “which object occupies which role.”

By contrast, anti‑haecceitists (or “qualitative supremacists”) deny that there are such additional, non-qualitative facts. For them, once all qualitative facts are fixed, there is no further metaphysical difference between possibilities.

Haecceitism in Modal Metaphysics

In contemporary analytic philosophy, haecceitism is often framed using possible worlds semantics for modality (possibility and necessity).

De re modality and transworld identity

A central question is whether an individual—say, Socrates—can be said to exist in more than one possible world in a primitive sense. Haecceitists typically affirm primitive transworld identity: there is a fact of the matter that the individual in world W1 is the very same entity as that in world W2, not just a counterpart related by similarity.

This contrasts with David Lewis’s counterpart theory, which is anti‑haecceitist. For Lewis, individuals are worldbound; cross-world statements about Socrates (“Socrates could have been a carpenter”) are made true by similar counterparts in other worlds, not by one numerically identical Socrates across worlds. Lewis explicitly rejects additional, irreducible “thisness” facts.

Haecceitistic differences between worlds

A standard way of formulating haecceitism is in terms of haecceitistic differences: differences between possible worlds that do not involve any variation in qualitative facts, only in “which individual occupies which role.”

Example: Suppose there are two qualitatively identical individuals, A and B, and two roles, left and right, in a symmetric arrangement. Consider:

• World W1: A is on the left, B on the right.
• World W2: B is on the left, A on the right.

Qualitatively, the two worlds are structured identically: two indistinguishable objects, leftness, rightness, same distances, same masses, etc. The only difference is a non-qualitative one about which individual occupies which position.

• Haecceitists say W1 and W2 are distinct possible worlds, differing in primitive identity facts.
• Anti‑haecceitists say there is no real metaphysical difference between W1 and W2; they are, in some deep sense, the same possibility described in two ways.

This issue also arises for permutations of qualitatively identical particles in physics, Label‑switching in mathematics, and debates over structuralism in metaphysics and philosophy of science.

Arguments For and Against Haecceitism

Debate over haecceitism centers on whether positing non-qualitative thisnesses is metaphysically acceptable or necessary.

Motivations and arguments for haecceitism

1. Intuitive de re possibilities
   Proponents claim that ordinary modal talk seems to presuppose primitive identity: we say this very person could have had a different career, appearance, or location. Such talk, they argue, is more naturally interpreted as involving the same individual across multiple worlds, not just similar counterparts or purely qualitative descriptions.

2. Fine-grained individuation of possibilities
   Haecceitism allows a finer-grained space of possibilities. It distinguishes scenarios that are qualitatively the same but differ in “who is who.” Supporters argue this better matches our reasoning about permutations, re-identification, and personal identity.

3. Scotus and the problem of individuation
   Historically, Scotus’s idea of haecceitas was designed to explain numerical distinctness between two qualitatively indiscernible entities (e.g., two angels, or two electrons on a classical conception). Contemporary haecceitists adapt this thought: if qualitative profiles do not fully account for individuation, some primitive factor—thisness—must complete the story.

4. Non-qualitative properties and names
   Some philosophers argue that proper names, singular terms, or singular concepts appear to latch onto individuals directly, not via qualitative descriptions. Haecceitism promises a metaphysical ground for this directness in non-qualitative identity facts.

Criticisms and arguments against haecceitism

1. Ontological extravagance and mystery
   Critics contend that postulating irreducible “thisnesses” is metaphysically costly and obscure: what are such entities, how are they known, and what work do they do beyond duplicating qualitative facts?

2. Qualitative sufficiency and anti‑haecceitism
   Anti‑haecceitists argue that all genuine differences between possibilities are ultimately qualitative differences. If W1 and W2 differ only by a label-swap, it is more parsimonious to regard them as the same world described under different names.

3. Symmetry and permutation arguments
   In physics and mathematics, symmetry considerations often support anti‑haecceitism. Permutations of indiscernible particles or elements are frequently treated as gauge or redundant—not new states or structures. Anti‑haecceitists use this to argue that haecceitistic differences lack physical or structural significance.

4. Lewisian counterpart theory
   Lewis’s influential framework provides a way to account for ordinary modal discourse without primitive transworld identity. If counterpart theory can capture all legitimate modal judgments, then, critics assert, haecceitism becomes explanatorily idle.

5. Epistemic access worries
   If haecceitistic differences are non-qualitative, some argue they are in principle undetectable: no possible observation or experience could distinguish W1 from W2 in the permutation example. This raises questions about how we could ever have epistemic access to haecceitistic facts.

Contemporary Significance

Haecceitism continues to play a role in several live debates:

• Metaphysics of individuals: It frames questions about what makes an entity the very individual it is, especially in cases of qualitative indiscernibility, duplicate worlds, and perfect symmetry.
• Modal logic and semantics: Positions regarding haecceitism influence how philosophers design logical systems for de re modality, the treatment of rigid designators, and cross-world identity conditions.
• Philosophy of physics: Discussions of identical particles, permutation invariance, and spacetime points often hinge on whether one adopts a haecceitist or anti‑haecceitist stance about the underlying ontology.
• Structuralism and identity: In metaphysical and mathematical structuralism, some argue that only structural or relational facts are fundamental. Haecceitism is then cast as a rival view insisting on an additional layer of non-structural identity facts.

Across these contexts, haecceitism functions less as a fully developed theory than as a constraint or choice point: whether to recognize primitive facts about “who is who” that go beyond all qualitative structure. Its acceptance or rejection shapes how fine-grained one takes modal reality to be and what one counts as a genuine difference between possible worlds.