Many decisions of the boards of appeal warn against an ex post facto approach when assessing inventive step (see also the Guidelines G‑VII, 8 – November 2018 version). This applies especially to inventions which at first sight seem obvious, to combination inventions and where the proposed solution is supposedly "simple". Correct application of the problem and solution approach avoids this inadmissible ex post facto analysis which draws on knowledge of the invention (T 24/81, OJ 1983, 133; T 564/89; T 645/92; T 795/93).

When assessing inventive step, an interpretation of the prior art documents as influenced by the problem solved by the invention, where the problem was neither mentioned or even suggested in those documents, must be avoided, such an approach being merely the result of an a posteriori analysis (T 5/81, OJ 1982, 249; T 63/97; T 170/97; T 414/98).

In T 970/00 the board stated that any ex post facto analysis, and in particular any conclusion going beyond what the skilled person would have objectively inferred from the prior art, without the benefit of hindsight knowledge of the invention, is of necessity at variance with a proper application of the problem and solution approach. Any attempt to interpret the disclosure of the closest prior art so as to distort or misrepresent, based on hindsight knowledge of the invention, the proper technical teaching of the disclosure in such a way that it artificially meets specific features recited in the claim under consideration must fail, especially as this would risk unfairly and tendentiously concealing the technical contribution of the invention and prejudice the subsequent objective determination of the technical problem solved by the claimed invention (see also T 266/07, T 1486/10).

In T 2201/10, the board found that the examining division had taken an ex-post-facto approach to analysing the facts. Even assuming that the proposed solution could be derived from common general knowledge, it went against the very essence of the teaching in the closest prior art and, therefore, would not realistically have been chosen. The problem and solution approach required that documents not relating to the technical field of the invention be disregarded, and it also precluded any analysis according to which the skilled person would have modified the closest prior art in a manner contrary to its very purpose. In other words, a finding that a claimed invention deviated from what was, in light of the aim pursued, the essence of a prior-art disclosure was enough by itself to conclude that it was inventive over that prior art (see also T 2057/12).

In T 855/15 the board held that for the assessment of obviousness no argument was required as to whether the skilled person would select a document. A consideration as to whether the skilled person would select a particular document in order to arrive at the invention as claimed would amount to hindsight reasoning, because the skilled person would have to be assumed to know the invention. The board in T 2057/12 agreed with the board in T 855/15 insofar as the closest prior art belonged to the same or a neighbouring technical field of the skilled person or to his common general knowledge. Argumentation or evidence should be provided as to why the skilled person in a specific technical field would have indeed envisaged selecting a document in a remote field of technology as closest prior art or whether he would indeed have considered adapting a prior-art disclosure originating from his technical field to implement it in a remote technical field.