Describing Wholes and Parts#

Introduction#

One of the central ideas in EMMO is that everything in the physical world can be described as a whole composed of parts. This view is called the holistic perspective, and it is grounded in mereology — the study of part–whole relations.

In this perspective:

  • A whole is anything that exists as a unified entity.

  • Parts are the entities that collectively make up the whole.

For example:

  • A battery cell consists of electrodes, electrolyte, separator, and case.

  • Each electrode may consist of a coating and a current collector.

  • A coating may itself contain active material, binder, and conductive additive.

By describing systems in terms of their parts, EMMO provides a clear, logical structure for representing complex physical objects across scales — from atoms to full devices.

The Holistic Perspective in EMMO#

EMMO models mereological relations using the general property hasPart and its inverse isPartOf. This structure allows any physical entity to be expressed as a hierarchy of parts, enabling queries and reasoning such as:

  • “What are the parts of this object?”

  • “Which systems contain this component?”

  • “If object A is part of B, and B is part of C, then A is part of C.”

Because mereological relations are transitive, reasoning engines can infer part–whole relationships several levels deep.

Holistic structure example

Example of a hierarchical part–whole structure: a cell as a whole composed of parts at multiple levels of granularity.#

Using hasPart#

The most general way to describe composition in EMMO is with emmo:hasPart.

  • hasPart expresses that the subject contains the object as one of its physical parts.

  • isPartOf is its inverse, expressing that the subject belongs to the object.

This is the safest relation to use when the composition type is unknown or unspecified. It works at any scale and in any domain.

{
  "@context": "https://w3id.org/emmo/domain/electrochemistry/context",
  "@type": "PhysicalObject",
  "hasPart": [
    { "@type": "Electrode" },
    { "@type": "Electrolyte" },
    { "@type": "Separator" }
  ]
}

Here, the physical object (for example, a battery cell) is described generically as having three main parts. Using hasPart keeps the model flexible and broadly applicable.

Specialized Subproperties of hasPart#

While hasPart can be used universally, EMMO also defines several specialized subproperties that refine the meaning of part–whole relations. These subproperties add semantic precision, helping you describe how a part relates to the whole — whether as a material constituent, a functional component, or a member of a collection.

All of these are subproperties of ``hasPart``, meaning that if A hasConstituent B or A hasComponent B, a reasoning engine will also infer that A hasPart B. This ensures consistency while allowing for more precise modeling.

hasConstituent#

Definition: The relation between an object and one of its holistic parts that contributes to the object under some spatial-based criteria.

In practice, hasConstituent is used to describe material composition — the parts that occupy space within the whole and define what it is made of. These parts may blend or mix, and they do not necessarily maintain distinct boundaries.

Use hasConstituent when representing the material makeup of substances or mixtures.

Examples:

  • A polymer composite hasConstituent filler and polymer matrix.

  • An electrode coating hasConstituent active material, binder, and conductive additive.

  • A gas mixture hasConstituent oxygen and nitrogen.

{
  "@type": "ElectrodeCoating",
  "hasConstituent": [
    { "@type": "ActiveMaterial" },
    { "@type": "Binder" },
    { "@type": "ConductiveAdditive" }
  ]
}

hasComponent#

Elucidation: The relation between an object and one of its holistic parts that contributes to the structure or function of the object while retaining a distinct identity.

In EMMO, Component is a subclass of ``Constituent`` — every component is a constituent (it occupies space within the whole), but not every constituent is a component. Components are typically assembled or connected parts that work together to realize the behavior of the whole.

Use hasComponent for structural or functional subunits that can be identified individually.

Examples:

  • A battery cell hasComponent positive electrode, negative electrode, separator, and electrolyte.

  • A motor hasComponent rotor, stator, and shaft.

  • A sensor module hasComponent microcontroller and housing.

{
  "@type": "BatteryCell",
  "hasComponent": [
    { "@type": "PositiveElectrode" },
    { "@type": "NegativeElectrode" },
    { "@type": "Separator" },
    { "@type": "Electrolyte" }
  ]
}

hasMember#

Definition: The relation between individuals representing a collection and one or more of its members.

Unlike hasComponent or hasConstituent, hasMember does not describe physical composition but membership in a set, group, or ensemble.

Use hasMember when describing collections or batches of entities that share a collective identity but are not physically connected.

Examples:

  • A sample batch hasMember specimen 1, specimen 2, specimen 3.

  • A dataset hasMember file 1, file 2, file 3.

  • A battery module, treated as a collection, hasMember cell 1–cell n.

{
  "@type": "CellBatch",
  "hasMember": [
    { "@id": "ex:Cell_001" },
    { "@id": "ex:Cell_002" },
    { "@id": "ex:Cell_003" }
  ]
}

Domain-specific subproperties#

Domain ontologies extending EMMO often introduce their own domain-specific part relations for clarity and query efficiency. For example, in the electrochemistry domain:

  • hasElectrode — instead of a generic hasPart relation.

  • hasSeparator, hasElectrolyte, hasCurrentCollector — for defining well-known components of a cell.

  • hasCoating — to describe electrode architecture.

These are subproperties of hasPart that express physically meaningful relationships without losing generality. A reasoning engine can always infer that if A hasElectrode B, then A hasPart B.

When to choose which relation#

Scenario | Recommended Relation | Description |

|---------------|---------------------------|-----------------| | Describing blended or inseparable material composition | hasConstituent | Use when the whole is defined by the spatial presence of its material parts. | | Describing structural or functional subparts that retain identity | hasComponent | Use when parts are assembled, connected, or serve a distinct role in the whole. | | Describing collections or groups of similar entities | hasMember | Use for batches, datasets, or ensembles rather than physical assembly. | | You are working within a domain ontology with specific subproperties (e.g., hasElectrode) | Domain-specific property | Use the most precise available relation. |

Example: describing a battery cell#

Here’s how a battery cell could be represented using different levels of specificity:

Generic description (using ``hasPart``):

{
  "@type": "BatteryCell",
  "hasPart": [
    { "@type": "Electrode" },
    { "@type": "Electrolyte" },
    { "@type": "Separator" }
  ]
}

More specific (using domain relations):

{
  "@type": "BatteryCell",
  "hasElectrode": [
    { "@type": "PositiveElectrode" },
    { "@type": "NegativeElectrode" }
  ],
  "hasElectrolyte": { "@type": "LiquidElectrolyte" },
  "hasSeparator": { "@type": "MicroporousPolymerSeparator" }
}

Describing composition (using ``hasConstituent``):

{
  "@type": "ElectrodeCoating",
  "hasConstituent": [
    { "@type": "ActiveMaterial" },
    { "@type": "Binder" },
    { "@type": "ConductiveAdditive" }
  ]
}

These examples are semantically consistent because all specialized properties (hasElectrode, hasConstituent) are subproperties of hasPart. This means generic queries for hasPart will still return all relevant relationships, even when specialized terms are used.

Reasoning across part hierarchies#

The transitive nature of hasPart allows for automatic inference. For example:

::

If A hasPart B and B hasPart C, then A hasPart C.

This means a reasoning engine can deduce, for instance, that:

  • a battery cell hasPart active material, even if only indirect relations (via electrode and coating) were explicitly defined.

Best Practices#

  • Use hasPart when you need flexibility and broad compatibility.

  • Use hasComponent or hasConstituent when the distinction between structural assembly and material mixture matters.

  • Use domain-specific subproperties (e.g., hasElectrode, hasSeparator) when your ontology defines them — they improve clarity and make SPARQL queries easier.

  • Always ensure inverse relations (isPartOf, isComponentOf) are also correctly defined in your ontology.

  • Combine part relations with classification and properties to provide rich, semantically consistent descriptions.

Summary#

The holistic perspective in EMMO provides a structured way to represent complex systems as wholes composed of interrelated parts. It supports reasoning across scales, promotes reuse of modular definitions, and ensures semantic precision through hierarchical relations:

  • hasPart — general purpose.

  • hasComponent — structural assembly.

  • hasConstituent — material mixture.

  • Domain-specific subproperties (e.g., hasElectrode) — context-aware precision.

By adopting this structure, you can describe materials, devices, and processes in a way that is modular, queryable, and logically consistent with the EMMO framework.