To determine whether the claimed invention, starting from the closest prior art and the objective technical problem, would have been obvious to the skilled person, the boards apply the "could-would approach" (see also Guidelines G‑VII, 5.3 – November 2018 version). This means asking not whether the skilled person could have carried out the invention, but whether he would have done so in the expectation of solving the underlying technical problem or in the expectation of some improvement or advantage (T 2/83, OJ 1984, 265; T 90/84; T 7/86, OJ 1988, 381; T 200/94; T 885/97). When considering whether or not claimed subject-matter constitutes an obvious solution to an objective technical problem, the question to be answered is whether or not the skilled person, in the expectation of solving the problem, would have modified the teaching in the closest prior art document in the light of other teachings in the prior art so as to arrive at the claimed invention (T 1014/07, T 867/13). So the point is not whether the skilled person could have arrived at the invention by modifying the prior art, but rather whether, in expectation of the advantages actually achieved (i.e. in the light of the technical problem addressed), he would have done so because of promptings in the prior art (T 219/87, T 455/94, T 414/98).

In T 1014/07 the board stated that since this question involves determining whether or not the skilled person would – and not could – have made a particular modification, it is necessary for answering the question to identify conclusive reasons on the basis of tangible evidence that would have prompted the skilled person to act in one way or the other. The board in T 1045/12 found that its decision, being based on prior-art documents D4 and D3, was based on "tangible evidence" (referring to T 1014/07).

It has been held that once an invention existed, it could often be shown that the skilled person could have made it by combining different elements in the prior art, but such arguments had to be disregarded as the product of ex post facto analysis (T 564/89).

According to T 939/92 (OJ 1996, 309), the answer to the question what a skilled person would have done depended in large measure on the technical result he had set out to achieve. In other words, the notional "person skilled in the art" was assumed to act not out of idle curiosity but rather with a specific technical purpose in mind.

In T 867/13 the board pointed out that what the skilled person, starting from the closest prior art and faced with the objective technical problem, would or would not do depends not solely on the disclosure of the closest prior art document, but also on the state of the art in the relevant technical field.

In T 1126/09 the board pointed out that, in accordance with the "could-would" approach, the assessment of inventive step must involve establishing, in each individual case, to what extent the skilled person had good reason, in the light of the closest state of the art or the objective problem derivable from it, to adduce further prior art and apply its teaching to the process/apparatus of the closest prior art – or, in other words, whether any factor is discernible which points towards a combination of the teachings of the citations addressed.

Technical feasibility and the absence of obstacles were only necessary requirements for reproducibility but were not sufficient to render obvious what was actually achievable for the skilled person (T 61/90). The fact that the inherent properties of a technical means were known to the skilled person, so that he had the intellectual possibility to apply this means in a conventional device, merely established the possibility of using such technical means in such a manner, i.e. that the skilled person could have used it. However, if it was to be established that such intellectual possibility was also a technical measure which it was obvious for the skilled person to use, it was necessary to show that there was a recognisable pointer in the state of the art to combine the known means and conventional device for achieving the intended technical aim, i.e. that the skilled person would have made such a combination. The existence of such a technical reason was dependent on the known properties not only of the means but also of those of the device (T 203/93, T 280/95). That it was theoretically possible for the skilled person to arrive at the invention simply means that he could have used the requisite technical means. If, however, it is to be established that he would actually have used them, it must be possible to ascertain a pointer in the prior art which would have prompted him to do so (T 1317/08).