The first step is to consider the particular context within which the design will have to work. As usual, we will introduce the idea by as elementary an example as possible. You are asked to select a two-way air valve to perform a particular function and you say that you cannot do so until you know where it is to go. If it is to go into a steel mill your design must be quite different from one needed, for instance, to operate a dentist's chair. In a steel mill no one ever turns a valve on; they hit it. They snatch up the heaviest blunt instrument, generally a spanner or crow-bar, and you may well have to choose the valve with these knock-out blows as a major design consideration. For doing the identical job in a dentist's chair you need only have a little valve with a knob suitably polished to avoid scratching the fingernails of his glamorous assistant. The context is decisive.

Now although concrete working conditions are part of the context they are not necessarily the whole of it. Abstract considerations such as cost, reliability, accessibility, appearance, and so on, may well have something to say about which design we select.

In short, we construct a framework of relevant considerations and assess each design by how neatly it fits in.