Eidos (pronounced ahy-dos) means "form" or "essence". It is an idea attributed to Plato and Aristotle.
A simple google search will reveal that Eidos, as it is with all ancient Greek ideas, is an extensively interpreted idea. I am not an expert in all those interpretations. I like the one from John Vervaeke, from his YouTube series, Awakening from the Meaning Crisis. As per John, when properly understood, Eidos is a framework to build deeper understanding of the real world.
Let us start with an example.
Suppose you want to know what a chair is. Should you understand that from someone who can only describe what a chair is? Or should you rather find someone who knows how to build a good chair? A good describer has propositional knowledge about the chair, which he or she often outlines as function performed. A good builder has the "form" or "essence" in his or her mind, which they leverage to actualize wood (matter) into a chair (product).
It is this “essence” that John explains as Eidos. He refers to it as structural-functional organization of an idea, which is different from and more than just a list of functions. You should always go to a builder to understand what a chair is because a good builder understands both structure and function.
Consider your favorite cup you drink coffee from in the morning. You can describe it propositionally - as something that enables you to drink coffee from in the morning. Or you can engage with it to observe how structure and function interact dynamically. By noticing how the handle of the cup is perfect for the size of your hand. How the size of the cup allows you to pour the optimal amount of coffee you want to drink in the morning. By observing how it mixes the aroma with the taste of your sip of coffee. Through this observation, you can know how structure and function interact.
John calls this type of knowing, Participatory Knowing. It is this principle of Eidos and Participatory Knowing that helps build deeper understanding of real world.
Eidos, therefore, teaches us that our perception of functional reality is intertwined with our understanding of structural reality of the world. Because structure and function always co-evolve in complex and dynamic real systems. Cell structure and cell biology are interdependent. Social institutions and societies co-evolve.
Just a list of functions or requirements doesn’t make a real world product. Good product can only be designed with a deep and adaptive understanding of the structure of real world it is going to operate in.
Insofar builders understand reality, and the essence of real problems through real world continuous participation and engagement, they can actualize the underlying solution idea into meaningful real world products.
In “software eats the world” paradigm, where we increasingly deploy technology to solve complex real world problems, keeping this principle of Eidos in the foreground can help builders to create meaningful technology to enable human agency and purpose.