Studying the vast classes and IRIs of EMMO is already a difficult task when we first encounter it. The inconvenient news is that we are soon asked to face an even deeper difficulty. We have to begin by setting aside, at least for a moment, the familiar way in which we have handled materials data. Quite naturally and almost by convention, we tend to start from the perspective of a single material or specimen. We then attach composition, processing conditions, property values, and measurement results around it in a way that feels very familiar. However, if we want to understand EMMO in a more essential sense, we need to stop seeing a material merely as one record and instead view it as something that exists, changes, and forms relations with other entities in time and space.
The Familiar Perspective of a Material-Centered Record
Conventional materials data modeling usually places one material or specimen at the center. Around that center, we add data such as composition, processing conditions, property values, measurement results, images, and literature information. This approach is highly intuitive when organizing experimental data or building a database. There is one material record, and the values and descriptions related to that record are connected one after another. Many research databases have been built in this way.
However, the world that EMMO tries to describe is based on a substantially different viewpoint. EMMO does not begin by placing one particular material entity at the center and treating all other information as its subordinate attributes. Instead, it tries to represent a world of multiple entities that coexist, change, and form relations with one another within the four-dimensional world of time and space.
Not Only Materials, But Also the Things That Exist Together
In the world of EMMO, a material is not the only axis around which everything else revolves. When a material is made, processed, measured, and interpreted, many different entities appear together, and EMMO asks us to pay attention to all of them. Raw materials, instruments, processes, processing conditions, environments, microstructures, defects, interfaces, measurement devices, measurement actions, measurement results, data files, equations, models, simulation processes, and computational results can all be included in that world and understood as coexisting entities.
The important point is that these entities do not remain merely as information related to a material. Some are actual physical objects. Some are processes that unfold over time. Some are observational results obtained through those processes. Others do not directly point to the actual object itself, but serve as representations of that object, such as data, equations, images, or models. In EMMO, these targets are identified as distinguishable entities when necessary and are understood as beings that enter into different kinds of relations.
At this point, what matters is not simply connecting many items. We need to distinguish what is part of what, what participated in which process, what happened earlier in time, what caused or contributed to a change, and what functions as data or a sign representing an actual object. The fact that two things are related to the same material is not sufficient. The type of relation must also be made visible, whether it is a part-whole relation, a process participation relation, or a relation between observation and representation.
Mereotopology: Looking at Parts and Connections Together
This is where one of the important foundations of EMMO appears: the perspective of mereotopology.
Mereotopology combines mereology, which deals with what is part of what, and topology, which deals with how things are connected, share boundaries, or come into contact. In simple terms, it is a framework for explaining how entities do not exist as isolated points, but become parts of one another, form boundaries, touch one another, and constitute a larger whole.
For example, a single specimen may have phases, grains, defects, and interfaces as its parts. A grain can be part of the specimen while also serving as an element that constitutes its microstructure. An interface can be understood as the boundary where two grains or two phases meet. A heat treatment process is a process in which the specimen, equipment, atmosphere, temperature conditions, and time conditions participate together. A measurement is another process in which the specimen, measurement device, measurement conditions, and observer enter into a specific relation.
Part-Whole Relations Are Not Simple Ownership Relations
Therefore, in EMMO, a part-whole relation is not a simple relation of data ownership. It means more than a mechanical connection such as “this measured value belongs to this material record,” or an inclusion relation defined by convention inside a database. In other words, EMMO asks what entity is a physical part of another entity in the actual world, which entities share a boundary, which entities participate together in a process, and which state can be understood as one phase within a longer temporal change.
If we take a secondary battery as an example, an electrode may look like one material entity. From that perspective, active materials, binders, conductive additives, current collectors, pore structures, and interfaces may appear to be secondary attributes that constitute the electrode. From the viewpoint of mereotopology, however, they should be understood not merely as lower-level attributes, but as distinguishable entities that constitute the electrode as a whole, or as entities that contact one another and form boundaries. Active material particles may be parts of an electrode layer, the electrode layer may be part of the electrode, and the interface where the active material meets the electrolyte may become an important boundary where electrochemical reactions occur.
Part-whole relations also do not remain limited to spatial composition. If we remember that EMMO adopts a four-dimensional viewpoint, temporal parts must also be considered. A specimen before fabrication, a specimen during heat treatment, a specimen at the time of measurement, and a specimen after degradation are obviously not unrelated objects. At the same time, they are not the exact same object in the same state. They can be understood as different phases through which one specimen passes in time, or as temporal parts that constitute the entire life of the specimen.
From this perspective, every distinguishable entity can become a center in its own context. Put differently, each of them can become a protagonist. A specimen may be an output or a processing target in a manufacturing process, but it becomes an object of observation in a measurement process. A measurement device may be a participant in measurement, but in a calibration process the device itself becomes the object of analysis. Data may be a representation of a measurement result, but in a model training process it becomes input data, and in a paper or database it becomes a knowledge resource that other researchers must interpret again.
One entity can be the protagonist in one relation and a supporting actor that explains another entity in a different relation. What matters is not to fix one entity permanently at the center, but to accept that the center can move according to the context of the relation. EMMO tries to deal with the way many entities constitute the world from their own positions, become parts of one another, participate in processes, represent objects, and form a meaningful structure together.
Making Complexity Understandable Without Removing It
EMMO does not allow this complexity to remain merely as an arbitrary collection of links. It is not enough to say that materials, processes, measurements, data, and models are related to one another. We need to distinguish whether the relation is a part-whole relation, a contact or boundary relation, a process participation relation, a temporal precedence relation, a causal connection, or a relation between an actual object and its representation.
For example, the sentence “heat treatment conditions are related to the grain size of a specimen” may appear to be a simple connection on the surface. From the viewpoint of EMMO, however, this connection can be divided into more precise parts. Heat treatment is a process that unfolds over time. The specimen is a physical object that participates in that process. Temperature and time act as conditions of the process. Grain size is a property related to the microstructure of the specimen after heat treatment, and its value is obtained again through a specific measurement process and data representation.
In this way, materials knowledge that can be expressed in a single sentence is, in reality, formed by overlapping entities and relations. What matters is that it should be represented in that way. A process changes an object, measurement observes the changed object, data records the observational result, and a model interprets or predicts the phenomenon in a specific way. EMMO does not compress all of this into a one-dimensional explanation. It tries to distinguish the roles and relations of each element.
EMMO can therefore be understood as a system that organizes complexity in a mutually understandable way while preserving it as much as possible. It is not simply about creating many nodes and drawing many lines. Rather, it aims to build a detailed structure that clarifies what each connection means and helps make interpretation more precise.
In the conventional RDBMS approach, where a large table is built to describe a material, much information tends to gather around one record. In EMMO, by contrast, the whole world in which a material is made, changes, is measured, and is represented by data and models is reconstructed as a network of smaller entities and relations. The important point is not to push the material out of the center, but to open a broader context for the world in which the material actually exists.
This flexibility of EMMO does not mean that any relation can be freely connected in any way. In fact, it is closer to the opposite. EMMO expresses complex relations while preventing us from losing what those relations mean. In that sense, EMMO can be described as an overall semantic framework that preserves the complexity of materials data and enables different data and knowledge to be connected without damaging that complexity.
How Materials Data Becomes Knowledge
Using EMMO should not begin from the idea that we simply need to study and apply every concept in a vast top-level ontology. It should begin from moving beyond that expectation. More importantly, materials researchers need to ask again what the specimen, process, measurement, property, data, and model that they already handle actually mean, and they need to examine carefully how these things exist and relate to one another within the world.
In summary, the philosophy of EMMO allows us to move away from the conventional approach of merely attaching more items to the data of the material of interest. From a four-dimensional viewpoint, where both space and time serve as axes, EMMO can be understood as a lens that helps us look in greater detail and with greater rigor at the process through which materials data becomes practically useful knowledge.