Identity
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Identity | |
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| Type | Metaphysical concept |
| Field | Metaphysics; Ontology |
| Core idea | Conditions under which an entity is the same entity over time or across descriptions |
| Assumptions | Entities can be tracked across change; criteria of sameness can be meaningfully stated |
| Status | Conceptual |
| Related | Ontology; Personal identity; Persistence; Causation |
Identity is a metaphysical concept concerned with the conditions under which an entity is the same entity over time, across change, or under different descriptions. Questions of identity address what it means for something to remain itself despite alteration, and how entities are distinguished from one another.
Identity plays a central role in metaphysics and ontology, with implications for philosophy of mind, ethics, law, and scientific explanation.
Core idea
At its core, identity concerns sameness. To claim that an entity at one time is identical to an entity at another time is to claim that they are not merely similar, but numerically the same entity.
Identity is typically contrasted with qualitative similarity. Two objects may share all observable properties yet still be numerically distinct, while a single object may change properties over time while remaining the same entity.
Identity over time
A major focus of identity theory is persistence through change. Objects gain and lose properties, parts, and relations, yet are often treated as continuing entities.
Philosophical accounts differ on what grounds this persistence, including continuity of material composition, causal continuity, functional organization, or relational structure.
Criteria of identity
Criteria of identity specify the conditions under which identity holds. Such criteria may be:
- Temporal — relating entities across time.
- Modal — relating entities across possible scenarios.
- Contextual — varying by explanatory or practical context.
Disagreement often arises over whether identity criteria are absolute or context-dependent.
Personal identity
One influential area of discussion concerns personal identity. Philosophers ask what makes a person the same person over time, especially in cases involving memory loss, psychological change, or bodily alteration.
Proposed accounts emphasize psychological continuity, bodily continuity, or narrative coherence, each with distinct implications.
Identity and ontology
Identity is closely tied to ontological categorization. Whether two descriptions refer to the same entity depends on how entities are individuated within a given framework.
Ontological assumptions about objects, processes, or structures shape how identity claims are evaluated.
Identity in science
Scientific theories rely on identity conditions when tracking entities across observations and models. Examples include identifying particles, organisms, or systems across time and experimental contexts.
Disputes about identity in science often reflect deeper questions about what kinds of entities a theory commits one to.
Limits and problems
Some philosophical positions question whether strict identity applies universally. In cases involving vague boundaries, gradual change, or overlapping entities, identity claims may become indeterminate.
These problems motivate alternative approaches that emphasize relations, processes, or functional roles rather than strict numerical identity.
Status
Identity is a foundational metaphysical concept rather than a resolvable hypothesis. Its analysis clarifies assumptions about persistence, individuation, and sameness that underlie both everyday reasoning and theoretical frameworks.