 The fourth category concerns the quantitative knowledge just referred to, and the third the theoretical tools used to acquire it. These two categories can be assumed to match Bunge’s notion of substantive technological theories. The status of the remaining four categories is much less clear, however, partly because they are less familiar, or not at all, from the well-explored context of science. Of these categories, Vincenti claims that they represent prescriptive forms of knowledge rather than descriptive ones. Here, the activity of design introduces an element of normativity, which is absent from scientific knowledge. Take such a basic notion as ‘operational principle’, which refers to the way in which the function of a device is realized, or, in short, how it works. This is still a purely descriptive notion. Subsequently, however, it plays a role in arguments that seek to prescribe a course of action to someone who has a goal that could be realized by the operation of such a device. At this stage, the issue changes from a descriptive to a prescriptive or normative one. An extensive discussion of the various kinds of knowledge relevant to technology is offered by Houkes (2009).