a) General principles 

According to the established case law of the boards of appeal, the assessment of inventive step is to be made at the effective date of the patent on the basis of the information in the patent together with the common general knowledge then available to the skilled person (G 2/21, OJ 2023, A85; see also e.g. T 1329/04, T 1545/08, T 1433/14, T 488/16, T 1322/17).

It is often necessary to provide post-published evidence (i.e. evidence that was not public before the filing date of the patent in suit and was filed after that date) as proof that the problem has been solved (i.e. that the alleged technical effect is actually achieved). This applies in particular when the objective technical problem has to be reformulated, e.g. with regard to a closest prior-art document previously unknown to the patent proprietor (T 116/18 of 11 October 2021).

In the view of the board in T 116/18 of 11 October 2021 there were, where the proof that a problem had been solved rested on such post-published evidence, three diverging lines of case law from the boards regarding the circumstances under which this evidence could or could not be taken into account. These encompassed: a strict application of the "ab initio plausibility" standard, according to which post-published evidence could be taken into account only if, given the application as filed and the common general knowledge at the filing date, the skilled person would have had reason to assume the purported technical effect to be achieved; the "no plausibility" standard which seemed to reject the concept of plausibility; and the "ab initio implausibility" standard, according to which post-published evidence could only be disregarded if the skilled person would have had legitimate reasons to doubt that the purported technical effect would have been achieved on the filing date of the patent in suit. In a referral pursuant to Art. 112 EPC, the board asked the Enlarged Board in particular whether the standards "ab initio plausibility" or "ab initio implausibility" should be applied.

The Enlarged Board in G 2/21 (OJ 2023, A85) departed from the concept of "plausibility" however. It held that the term "plausibility" that is found in the case law of the boards of appeal and relied upon by the referring board (T 116/18 of 11 October 2021) did not amount to a distinctive legal concept or a specific patent law requirement under the EPC, in particular under Art. 56 and 83 EPC. It rather described a generic catchword seized in the jurisprudence of the boards of appeal, by some national courts and by users of the European patent system (point 92 of the Reasons). The Enlarged Board ruled that:

"I. Evidence submitted by a patent applicant or proprietor to prove a technical effect relied upon for acknowledgement of inventive step of the claimed subject-matter may not be disregarded solely on the ground that such evidence, on which the effect rests, had not been public before the filing date of the patent in suit and was filed after that date.

II. A patent applicant or proprietor may rely upon a technical effect for inventive step if the skilled person, having the common general knowledge in mind, and based on the application as originally filed, would derive said effect as being encompassed by the technical teaching and embodied by the same originally disclosed invention."

Applying these principles to the case law of the Boards of Appeal the Enlarged Board had analysed in reaching its decision, the Enlarged Board was satisfied that the outcome in each particular case would not have been different from the actual finding of the respective board of appeal. Irrespective of the use of the terminological notion of plausibility, the decisions appeared to show that the particular board of appeal focussed on the question whether or not the technical effect relied upon by the patent applicant or proprietor was derivable for the person skilled in the art from the technical teaching of the application documents.