IDEF5 Reference
The IDEF5 ontology description notation
What is IDEF5?
IDEF5 (Integrated Definition for Ontology Description Capture) is a formal graphical language for modelling ontologies: systematic accounts of the kinds of things that exist in a domain, the properties those things have, and the relationships between them.
IDEF5 was developed as part of the U.S. Air Force ICAM (Integrated Computer-Aided Manufacturing) initiative and was standardised in the 1990s. It is one of a family of IDEF modelling methods (IDEF0, IDEF1X, IDEF3, etc.), each targeting a different aspect of system description.
An IDEF5 ontology schematic answers questions like:
- What kinds of things exist in this domain?
- What essential properties define each kind?
- How are kinds related to one another — by taxonomy, composition, or association?
- Which things are specific individuals (named instances) rather than general categories?
- How do things in this domain change state over time?
- What processes act on the things in this domain?
IDEF5 is well-suited to knowledge engineering for expert systems, formal specification of domain models, and defining deterministic knowledge graphs for AI systems. It is more rigorous than informal concept maps, but less dense than description logics like OWL.
Ontology fundamentals
Before using IDEF5, it helps to be clear on a few philosophical distinctions the notation takes seriously.
Kinds vs. individuals
A kind is a category — a type of thing, not a specific thing. Dog, Metal, and Vehicle are kinds. A kind is not just any set of objects; it is a natural category defined by essential properties shared by all its members. Water is a kind because all water shares defining molecular properties. "Things I have seen today" is a set, not a kind.
An individual is a specific, named instance of a kind. Bolt #42 is an individual of kind Bolt. The Pacific Ocean is an individual of kind Ocean. Individuals have all the properties of their kind plus their own particular properties.
Essential vs. accidental properties
IDEF5 is concerned with essential properties — those that a thing must have in order to be what it is. Being H₂O is essential to being Water. Being cold is not essential: water can be warm. Accidental properties belong to specific instances or situations, not to the kind itself.
Relations vs. connections
In IDEF5, a relation is a semantic claim about a domain — a real-world association between kinds or individuals. It has ontological content: "Part contains Ink Supply" says something true (or proposed to be true) about the domain. A connection symbol (Forward, Backward, Plain) is a diagrammatic device that links process nodes and state nodes. It has no semantic content of its own; it just shows that one thing leads to another in a process flow.
Diagram types
IDEF5 defines three complementary diagram types. A complete ontology typically uses all three:
- Schematic diagram
- The primary type. Shows kinds, individuals, and the relations between them. Uses Kind circles, relation rectangles, and the taxonomic edge types (subkind-of, instance-of, part-of). This is what most Quiddity diagrams are.
- Elaboration document
- A structured text document that elaborates the properties and constraints of each kind identified in the schematic. Not directly represented in Quiddity's canvas, but the Notes field in the Properties panel is the appropriate place to record elaboration text for each node.
- Classification diagram
- A specialised schematic focused on taxonomic hierarchies — chains of subkind-of and instance-of relationships. Often a subset of a full schematic, laid out as a tree. Build these in Quiddity using Kind nodes and Subkind-of edges.
Reading an IDEF5 diagram
When reading an IDEF5 schematic, work through the elements in this order:
- Identify the kinds. Each circle is a kind. Read the labels to understand what categories exist in this domain.
- Read the taxonomy. Dashed lines with open arrowheads are subkind-of edges. They point upward (from subkind to superkind). Follow them to understand the classification hierarchy.
- Read the composition. Solid lines with arrowheads pointing to the "whole" are part-of edges. They show how kinds are assembled from components.
- Read the relations. Rounded rectangles are first-order relations. Each relation is labelled with the nature of the association. Edges connect the relation to the kinds it relates.
- Read the state transitions. Arrowed lines with midpoint circles show how things change state. Single arrowhead = possible (weak) transition; double arrowhead = necessary (strong) transition.
- Read process flows. Rectangular process nodes connected by Connect symbols and controlled by junction nodes show how processes sequence and branch.
Kinds & Individuals
Relations
Relations are associations between kinds or individuals that hold as a matter of domain fact. IDEF5 provides three notations for relations, depending on the arity (number of participants) and complexity of the relationship.
Process
Referent
State Transitions
State transition symbols model how things change over time. They connect a "before" state to an "after" state through a transition marker — the midpoint circle. The nature of the transition (possible vs. necessary) determines which symbol to use.
Junctions
Junctions control the logic of branching and merging in process flows. They express how multiple incoming or outgoing paths combine. All three junctions are depicted as circles distinguished by their interior mark.
Connecting Symbols
Connecting symbols are the "wires" of process and state flow diagrams. Unlike relation edges, they carry no semantic IDEF5 content of their own — they simply show that one element leads to another in a sequence or dependency. All three are connector tools in Quiddity.
Grouping & Annotation
Taxonomic edges
These three edge types express the fundamental ontological relationships between kinds and individuals in IDEF5. They are drawn by selecting a node, using a connection tool in Quiddity, and choosing the edge type in the Properties panel, or by setting the edge type when drawing.
| Edge type | Visual style | Meaning | Direction of arrowhead |
|---|---|---|---|
| Subkind-of | Dashed line, open triangular arrowhead | The source kind is a specialisation of the target kind. Every instance of the subkind is necessarily an instance of the superkind. The subkind inherits all essential properties of the superkind and has additional distinguishing properties of its own. | Points to the superkind (more general) |
| Instance-of | Dashed line, filled arrowhead | The source individual is a member of the target kind. The individual has all the essential properties of the kind. | Points to the kind |
| Part-of | Solid line, filled arrowhead | Mereological (compositional) relationship. The source is a part, component, or constituent of the target. This is a real-world compositional fact, not a subset relationship. | Points to the whole |
Relation edges
| Edge type | Visual style | Use |
|---|---|---|
| First-order | Solid line, filled arrowhead at target | Connects a first-order relation rectangle to each kind or individual it relates. The arrowhead points toward the node that plays the "range" (target) role. |
| Second-order | Solid line, filled triangle at source end | Connects a second-order relation to the first-order relation it is "about." The triangle tip points into the line at the source. |
| Alt. Relation (2-place) | Solid line, filled arrowhead, label on line | Compact notation for a binary relation. Used in place of a separate relation rectangle when the relation has exactly two participants. |
State transition edges
| Edge type | Visual style | Use |
|---|---|---|
| State Weak | Solid line, open midpoint circle, single arrowhead at right | Connects a "before" state kind through the midpoint transition circle to an "after" state kind. The transition is possible but not guaranteed. |
| State Strong | Solid line, open midpoint circle, double arrowhead at right | Same structure as weak, but the transition is necessary and irreversible once the preconditions are met. |
Process edges
| Edge type | Visual style | Use |
|---|---|---|
| Process Connect | Solid line, no arrowhead | Links process nodes, junctions, and state nodes in a process flow diagram. Shows that one element feeds into the next. Use Connect (Forward/Backward/Plain) symbols in Quiddity to create these edges. |
Full edge reference
| Edge type | Style | Meaning |
|---|---|---|
| First-order | Solid line, filled arrowhead at target | Connects a relation node to the kinds/individuals it relates |
| Second-order | Solid line, filled triangle at source | Connects a second-order relation to the first-order relation it describes |
| Subkind-of | Dashed line, open arrowhead pointing to superkind | Taxonomic specialisation |
| Instance-of | Dashed line, filled arrowhead pointing to kind | Individual is a member of a kind |
| Part-of | Solid line, arrowhead pointing to the whole | Mereological (compositional) relationship |
| State Weak | Solid line, midpoint circle, single arrowhead | Possible state change |
| State Strong | Solid line, midpoint circle, double arrowhead | Necessary and irreversible state change |
| Process Connect | Solid line, no arrowhead | Sequence or dependency between process elements |
| Alt. Relation | Solid line, filled arrowhead, label on line | Compact 2-place first-order relation |
| Connect Fwd | Solid line, forward chevron at midpoint | Directed process flow, source to target |
| Connect Bwd | Solid line, backward chevron at midpoint | Directed process flow, target to source |
| Connect Plain | Solid line, no directional mark | Undirected association in a process or state diagram |
Pattern: Kind hierarchies
The most common IDEF5 pattern is a taxonomic hierarchy: a tree of kinds connected by Subkind-of edges, possibly with individuals connected by Instance-of edges at the leaves.
Rules:
- Every kind at the bottom of a Subkind-of edge inherits all essential properties of the kind at the top.
- A kind can have multiple superkinds (multiple inheritance), but the model must be consistent — the kind must genuinely satisfy all the essential properties of all its superkinds.
- Subkind-of chains can be as deep as the domain requires. Avoid shallow hierarchies that just group things arbitrarily — only create a subkind when it has defining properties the superkind does not.
- Individuals at leaves connect to their immediate kind with Instance-of. There is no need to connect an individual to all superkinds — the Instance-of relationship is transitive through the Subkind-of chain.
Pattern: Part-whole composition
Part-of edges model how kinds are assembled from components. This is a compositional, not taxonomic, relationship.
Rules:
- Part-of is not transitive by default. If A is part of B and B is part of C, this does not automatically mean A is part of C unless you model that explicitly. Transitivity depends on the domain.
- A kind can have many parts, each connected by its own Part-of edge pointing to the whole.
- Part-of can be combined with Subkind-of: the same kind can be both a subkind of something and a part of something else. These are orthogonal relationships.
- Place a first-order relation rectangle on the connection when the compositional relationship needs a name or additional attributes (e.g. quantity, role).
Pattern: State diagrams
State transition diagrams model how a kind or individual moves between states over time.
Structure:
- Place the "before" and "after" states as Kind nodes.
- Draw a State Weak or State Strong edge from the before-state kind to the after-state kind. The midpoint circle appears automatically on the edge.
- To make the transition instantaneous, drag the Δ (Instantaneous Transition) symbol from the toolbox onto the state edge, or onto the midpoint circle, to mark it.
- Connect process nodes to the midpoint circle using Connect (Plain) edges when the transition is triggered by a process.
Weak vs. strong: Use Weak when the transition is conditional or probabilistic (water may freeze if temperature drops below 0°C under sufficient pressure). Use Strong when the transition is necessary and cannot be reversed (combustion of a fuel is irreversible).
Pattern: Process diagrams
Process diagrams show the sequence and branching of activities in a domain.
Structure:
- Place Process nodes for each activity.
- Connect them with Connect (Forward) or Connect (Plain) edges to show sequence.
- Insert Junction (AND/OR/XOR) nodes to model branching and merging of flows.
- Connect process flows to State Transition nodes where a process causes a state change.
Junction placement: Junctions are used in two roles — as splits (one incoming, multiple outgoing) and as merges (multiple incoming, one outgoing). Use the same junction type for a split and its matching merge: an AND-split should be merged by an AND-join, an XOR-split by an XOR-join.
Further reading
- IDEF5 Ontology Description Capture Method — official description at the Integrated DEFinition Methods (IDEF) website.
- The IDEF5 Method Report (1994), available from the IDEF organisation, is the authoritative reference for the full method including elaboration documents and classification documents.
- For background on ontology engineering more broadly, the Stanford Encyclopedia of Philosophy article on Logic and Ontology provides philosophical grounding for the concepts IDEF5 formalises.