# Domain-Driven Design: Why There Is No Universal Model > A model is never a faithful representation of reality, but an abstraction designed for a specific purpose. A model exists only through its use. - Author: Mathieu Eveillard - Published: 2025-08-27 - Tags: Modularity - Canonical: https://www.mathieueveillard.com/articles/ddd-modelisation/ - Translation (fr): https://www.mathieueveillard.com/fr/articles/ddd-modelisation.md ![Domain-Driven Design: Why There Is No Universal Model](https://www.mathieueveillard.com/articles/2025-08-27-ddd-modelisation/ddd-modelisation.webp) Software modeling exercise: If I asked you to model a table, what would you do? … Your first instinct should be to ask about the purpose. That’s because the model will differ depending on whether: - You’re an antique dealer and want to make sure this is indeed a Louis XV-era table; - You’re a surgeon and need to operate on a patient (the concept of cleanliness comes into play, and it’s not the same kind of cleanliness as that of the average person); - You are Vladimir Putin and want [to put your conversation partner at ease](https://en.wikipedia.org/wiki/Vladimir_Putin%27s_meeting_table). In these three cases, you’re not looking for the same qualities, so you’ll expect to check different boxes on your favorite e-commerce site. A good table, when you’re the head of the Kremlin, is at least 6 meters long, whereas a surgeon will be more interested in the material (steel, carbon fiber), whether it’s height-adjustable, and whether it hinders the staff’s movements. It goes without saying, but you have a specific table model in mind based on the use you intend. More generally, when you’re pursuing a goal, you implicitly construct a model of the world that serves that goal, and that goal alone: if you want to sail around the world, a map of ocean currents will be more useful to you than a map of religions or mineral deposits (on the other hand, a [topological](https://tafc.space/qna/the-topologists-world-map/) map might help). Moreover, it would make no sense to speak of “the” map of the world: no map can represent the entire world in all its dimensions. Reality is infinite, whereas the representation you create of it is necessarily finite. So what exactly is a model, in software terms? **A model is an abstraction, that is, a simplified representation of reality constructed for a specific operational purpose**. Traditionally, this is where we mention these two quotes: - “The map is not the territory” —[Alfred Korzybski](https://en.wikipedia.org/wiki/Alfred_Korzybski) - “All models are wrong, but some are useful” —[George Box](https://en.wikipedia.org/wiki/George_E._P._Box) Alfred and George are actually saying the same thing: **a model represents one facet of reality, not reality itself**. First and foremost, a model therefore leaves out all facets of reality except one. As for that one facet, it too is represented imperfectly: it may be simplified, incomplete, or downright wrong. And yet, this model can still prove useful for achieving your goal: a map, drawn to the right scale, will help you even if it isn’t perfectly up to date. When it comes to IT systems, this means **there is no single model, no “one-size-fits-all” model, and no canonical model**. Any attempt to do so would amount to mixing different purposes. There is one model per context—the term “context” here referring to the concept of bounded context in Domain-Driven Design—because a context precisely reflects an operational purpose. The goal is to distinguish between models so that they can diverge, each specialized for a very specific use. For example, your IT system cannot provide a single representation of a Product, because the different departments within the company do completely different things with that product: one designs it, another manufactures it, a third stores it, yet another markets it, and perhaps a final one recycles it. If you remember only one thing, make it this: ``` 1 context <=> 1 model ``` A model is therefore a projection of reality onto a use. But what, exactly, is reality? Can we truly access it, or do we only access its projections? This is, in essence, what Plato was already suggesting in [the Allegory of the Cave](https://en.wikipedia.org/wiki/Allegory_of_the_cave): humans have their backs turned to the Sun and see only the shadows of the real world cast onto the back wall of the cave. For these humans, the shadows _are_ the real world\*. _\* And the parallel ends here, for two reasons: 1. In DDD, there is no hierarchy; the models are sufficient for us, whereas Plato clearly states that shadows are worth less than full and complete reality; 2. But above all, Plato uses the allegory of the cave to illustrate the contrast between the sensible world and [the intelligible world](https://en.wikipedia.org/wiki/Theory_of_forms), the latter being defined precisely as the world of models, of pure forms. Drawing such a parallel may therefore be a misinterpretation._