Design is the practice of specifying things to be produced by others for others.
Let us start by defining the roles involved in all design.
Let us start by naming the role of a person for whom a design is specified and produced. Historically, this name has always been borrowed from other fields, always with distracting semantic baggage. User, audience, customer, consumer, persona and service actor have all at various times served this purpose. To clarify the field of design, it is necessary to coin a technical term of art, one signifying this role relative to design practice. Let us call any first-person subject for whom a thing is intended, specified and produced a designant.
Let us call the role of those who make or deliver the specified thing producers.
Finally, those who develop the specifications for the thing are designers.
And the specification of the thing is its design.
The specification is the result of myriad decisions made by designers, all aiming at a set of ideal goals. This is the change the designed thing is meant to have on reality. This is the design intention.
The defined intention of the work, plus the enabling resources and limiting constraints governing how the intention may be actualized, is the problem definition, or simply, the problem.
A design problem is a problem that includes within its definition considerations essential to design practice.
That is, a design problem includes within its definition not only the thing to be produced but the designants for whom the thing is intended.
A design methodology solves a design problem systematically, using techniques that address the particular challenge of designing for designants who are other than the designers who work on resolving the design problem.
Without inclusion of designants, a problem is not defined as a design problem.
Without design methodology, a design problem is not being approached as what it is.
Again: Design is the practice of specifying things to be produced by others for others. By this definition, a great many problems ought to be defined as design problems, but are not.
Further, a great many problems that are or ought to be defined as design problems are not approached using design methodology.
To use a simple analogy, a great many design problems are misdiagnosed as other kinds of problems; and many design problems, both incorrectly and correctly diagnosed, are treated incorrectly.
Things become messy almost immediately:
First, multiple roles can be played by the same person. In service design, for example, the service actors who deliver the service on the front lines, or who support the service behind the scenes, are also among those for whom the service is intended. They are both designants and producers. Another example is designing something for ourselves. If we design our kitchen, we may also do the installation and construction work and be producers, and, once we inhabit and use our new kitchen, we may also be among our own designants. But if we are alert, we will notice how our role shifts and how we think, feel and behave differently in each, often in surprising and conflicting ways.
Second, a designed “thing” can be many kinds of things, produced from many materials, some of which are not even physical substances — for example actions or concepts. And many design materials are complex and heterogeneous.
Finally, the nature of designed things is quite different from the objective way we normally think about things. This will require extensive rethinking, and will be a subject of discussion later. For now, suffice it to say, even the simplest designed things are in fact participatory systems. Designed things are not only for designants, they involve the designants as participants in the designed thing, and through their participation activate and complete it.

