 Choosing the design option that maximally meets all the functional requirements (which may but need not originate with the prospective user) and all other considerations and criteria that are taken to be relevant, then becomes the practical decision-making problem to be solved in a particular engineering-design task. This creates several methodological problems. Most important of these is that the engineer is facing a multi-criteria decision problem. The various requirements come with their own operationalizations in terms of design parameters and measurement procedures for assessing their performance. This results in a number of rank orders or quantitative scales which represent the various options out of which a choice is to be made. The task is to come up with a final score in which all these results are ‘adequately’ represented, such that the option that scores best can be considered the optimal solution to the design problem. Engineers describe this situation as one where trade-offs have to be made: in judging the merit of one option relative to other options, a relative bad performance on one criterion can be balanced by a relatively good performance on another criterion. An important problem is whether a rational method for doing this can be formulated. It has been argued by Franssen (2005) that this problem is structurally similar to the well-known problem of social choice, for which Kenneth Arrow proved his notorious impossibility theorem in 1950, implying that no general rational solution method exists for this problem. This poses serious problems for the claim of engineers that their designs are optimal solutions, since Arrow’s theorem implies that in most multi-criteria problems the notion of ‘optimal’ cannot be rigorously defined.