The closest prior art should relate to the same or at least a similar purpose (or objective) as the claimed invention. Even if prior art relating to the same purpose is available, it is not excluded that a document relating to a similar purpose might be considered to represent a better - or at least an equally plausible - choice of closest prior art, provided that it would be immediately apparent to the skilled person that what is disclosed in the document could be adapted to the purpose of the claimed invention in a straightforward manner, using no more than common general knowledge (Reasons, point 2.6).

If, despite the availability of prior art relating to the same purpose as the claimed invention (here: manufacturing a semiconductor substrate comprising a silicon-germanium film), it is nevertheless considered appropriate to select as closest prior art a disclosure relating to a similar purpose (here: manufacturing a semiconductor substrate comprising a germanium film), at least one claimed feature corresponding to the purpose of the invention will generally appear as a difference over the closest prior art (here: silicon-germanium).

However, this difference is not one which can legitimately be invoked in support of inventive step. The problem-solution approach presupposes that the skilled person has a purpose in mind from the very beginning of the inventive process, which in this case is the manufacture of a known type of semiconductor substrate comprising a silicon-germanium film. Within this conceptual framework, it cannot be logically argued that the skilled person would find no motivation to incorporate silicon-germanium. Moreover, an argument that it would not be straightforward to incorporate this difference into the teaching of the document considered to be closest prior art, or that this would require more than common general knowledge, would not, in such a case, constitute an argument in favour of inventive step, but rather an argument that this document is not in fact a promising starting point (Reasons, point 4.1).