R. 42(1)(c) EPC (former R. 27(1)(c) EPC 1973) stipulates that an application's description must "disclose the invention, as claimed, in such terms that the technical problem (even if not expressly stated as such) and its solution can be understood, and state any advantageous effect of the invention with reference to the background art". As long ago as T 26/81 (OJ 1982, 211), R. 27(1)(c) EPC 1973 was recognised as clearly binding. The boards' case law, and correct application of the problem and solution approach (see T 1/80, OJ 1981, 206; T 24/81, OJ 1983, 133), show that objective criteria must be used to determine the technical problem, i.e. the problem which can be seen to have been actually solved in the light of the closest prior art which may be different from the prior art which was at the disposal of the inventor (T 576/95). These objective criteria may be specifically defined by assessing the technical progress made in the subject-matter of the application against the closest prior art (T 20/81, OJ 1982, 217; T 910/90). A comparison of the problem indicated in the application with that indicated in a prior document must avoid an unduly abstract approach far removed from the practical thinking of the person skilled in the art (T 5/81, OJ 1982, 249).
In identifying the problem it is not permissible to draw on knowledge acquired only after the date of filing or priority. According to T 268/89 (OJ 1994, 50) the non-effectiveness of a prior art apparatus or method recognised or alleged only after the priority or filing date could not be drawn on in formulating the problem, particularly where that problem was adduced in support of inventive step in a "problem invention" (see T 2/83, OJ 1984, 265). Inventive step had to be assessed on the basis of the skilled person's knowledge before the priority or filing date (see also T 365/89).
For the purpose of the problem and solution approach, the problem must be a technical one that a skilled person in the particular technical field might be asked to solve at the priority date. The technical problem may be formulated using an aim which is to be achieved in a non-technical field, and which is thus not part of the technical contribution provided by the invention to the prior art (T 641/00, OJ 2003, 352; T 154/04, OJ 2008, 46); see also below point 9.1 "Treatment of technical and non-technical features".
In T 1639/07 the board held that the objective technical problem must be derived from physical, chemical etc. effects directly and causally related to the technical features of the claimed invention. An effect could not be validly used in the formulation of the technical problem if the effect required additional information not at the disposal of the skilled person even after taking into account the content of the application in question.
In T 87/08 the board stated that Art. 56 EPC requires that the assessment of inventive step is made "having regard to the state of the art". Accordingly, a decision is not sufficiently reasoned in the sense of R. 68(2) EPC 1973 (R. 111(2) EPC 2000) if the chain of reasoning to justify the finding of lack of inventive step merely states that a purported effect has not been achieved, i.e. this technical problem had not been solved, without reformulating the problem in a less ambitious way and without assessing obviousness of the claimed solution to that reformulated problem in the light of the cited prior art (T 1079/08, T 306/09, T 2375/10).