Design is concerned with how it works, and therefore, not so much with what is and what truly is (or truth).
So we can turn this concern on itself and ask: How does design work?
One answer to this question is a whole field of knowledge, namely “design methodology”, but we can find a rougher answer (and therefore more abstract) if we point out that the question itself is an application of design! We want to create stuff (not make stuff, there is a difference) and in order to do that we begin to concern ourselves with how stuff behaves when it is joined with other stuff.
Thus for example, designers’ love stories with the materials (“Didin’t you fall in love with wood?”). But thus also the designerly nerdness of obssessing over fonts, printing processes, spacing and negative space, and all the stuff that — exactly because they are being thus grouped — receive the name of techniques.
Also instructive is the (seemingly counter-intuitive) actual regard for everything that actually is technical. Like every designer has a reverence for the “mechanism” that allows tek-stuff to work, but at the same time all designers get narcolepsy when face with actual engineering — the envy causing in turn designers’ hate of engineers.
When we reapply further this questioning (something like how does the “how does it work?” of design work — or maybe even one layer more…) we get: Design is mostly the attempt of fitting together. Not only joining but joining in such a way that it is cool together. [Even though the word “cool” only meant cold back then in the prehistory of design, it is a very crucial concept that gets retroactively invented].
This is, of course, geometry. It is, also of course, why designers hate geometry — when the activity of fitting gets distilled into rules it gets stifled, it becomes brittle. Design is the process of fitting, if the perfect fit is automatically derivable by sines and arc-secants, then there is no process, there is only fit. But tragically the process of design necessarily produces rules as trying many times makes the designer aware of how eah format embodies many possible fittings.
The field of possible fits is, in a preverbal stage, the same as the “workings” of “how does it work?” And the “behaviour” of structuralism.
So design is also an epistemological stance — in a way, a child of some epistemological stances — and an approach to reality. Of course, as our explanation reaches such a level of abstraction, we lost the process. But that’s how it works.