The concept of plausibility goes to the heart of the fundamental principle that a monopoly for an invention can only be awarded in return for a disclosure of that invention.

The Boards of Appeal of the European Patent Office (EPO) and the courts of the United Kingdom have expressed the significance of plausibility as follows:

“The definition of an invention as being a contribution to the art, i.e., as solving a technical problem and not merely putting forward one, requires that it is at least made plausible by the disclosure in the application that its teaching solves indeed the problem it purports to solve.” ¹

“Plausibility is to exclude speculative patents, based on mere assertion, where there is no real reason to suppose that the assertion is true” ²

A lack of plausibility is not, in itself, a ground for invalidity of a patent according to the European Patent Convention or the UK Patents Act 1977. Rather, plausibility is usually raised in the context of sufficiency of disclosure or inventive step. The case law concerning plausibility appeared to be fairly settled, with a series of EPO Board of Appeal decisions finding that an invention must be rendered plausible in the application as originally filed, otherwise post-filing data cannot be considered in support of a sufficiency of disclosure or inventive step.

However, in a recent EPO Board of Appeal case (T 116/18) it was suggested that the Boards of Appeal are not completely consistent on this issue. In particular, it would appear that some Boards of Appeal consider that the requirement for the technical effect of a claimed invention to be rendered plausible in the application as filed is incompatible with the problem solution approach for assessing inventive step.

T 116/18 concerned European Patent No. 2484209, which was granted with claims directed to an insecticide comprising thiamethoxam and one or more compounds defined by a formula 1a encompassing a large number of possible compounds. The difference between the claimed subject-matter and the closest prior art was identified as the selection of the compounds to be combined, with the selected combination purported to have a synergistic effect.

The patent included examples testing the interaction between thiamethoxam and various compounds of formula 1a, but no evidence of synergy was provided. During appeal proceedings, the patentee provided post-published documents which contained data showing synergy between thiamethoxam and compounds of formula 1a. In response, the opponent, following what appeared to be the established EPO case law, argued that the synergy between thiamethoxam and compounds of formula 1a had not been rendered plausible in the application as filed and so the post-published documents should not be taken into account.

The Board of Appeal noted that the question of whether or not the post-published evidence could be considered was decisive for the question of whether or not the claimed subject-matter involved an inventive step. Then, somewhat surprisingly, the Board of Appeal identified conflicting earlier Board of Appeal decisions on the question of admissibility of post-published evidence to support inventive step.

The Board of Appeal referred to the requirement for a technical effect to be rendered plausible in the application as filed as “ab initio plausibility” and legal basis for this requirement was identified in a number of earlier decisions (including T 488/16, T 1329/04, and T 433/05).

However, they also referred to conflicting decisions in which the criterion for ab-initio plausibility had not been applied and where Boards of Appeal had found that a lack of ab initio plausibility was not a reason to disregard post-published evidence (for example, decisions T 2371/13 and T 31/18). These Boards of Appeal were of the opinion that the requirement for a technical effect to be plausibly demonstrated in application as filed was incompatible with the problem solution approach to assessing inventive step, because the problem to be solved is not known to the patentee until the closest prior art has been established, which may not happen until long after the application has been drafted and filed. As a result, the Boards of Appeal felt that the requirement of ab initio plausibility placed an undue burden on the applicant in the case of inventive step based on the problem solution approach.

According to the minutes of the oral proceedings that took place on 22 July 2021, all of the parties felt that there was a need to refer questions regarding the admissibility of post-published data to the Enlarged Board of Appeal (EBA). The written decision of the Board of Appeal was subsequently issued referring the following questions to the EBA:

If for acknowledgement of inventive step the patent proprietor relies on a technical effect and has submitted evidence, such as experimental data, to prove such an effect, this evidence not having been public before the filing date of the patent in suit and having been filed after that date (post-published evidence):

  1. Should an exception to the principle of free evaluation of evidence be accepted in that post-published evidence must be disregarded on the ground that the proof of the effect rests exclusively on the post-published evidence?
  2. If the answer is yes (the post-published evidence must be disregarded if the proof of the effect rests exclusively on this evidence), can the post-published evidence be taken into consideration if, based on the information in the patent application in suit or the common general knowledge, the skilled person at the filing date of the patent application in suit would have considered the effect plausible (ab initio plausibility)?
  3. If the answer to the first question is yes, can the post-published evidence be taken into consideration if, based on the information in the patent application in suit or the common general knowledge, the skilled person at the filing date of the patent application in suit would have seen no reason to consider the effect implausible (ab initio implausibility)?

It would seem that the second and third questions seek to establish whether rendering an effect plausible requires provision of a positive reason to believe that the alleged technical effect is achieved or the absence of a reason to doubt that the alleged technical effect is achieved. This leads on to the question of whether the burden of proof regarding ab initio plausibility rests with the patentee or the opponent. The Board of Appeal was also concerned whether it would be equitable to allow an opponent to rely on post-published evidence if the patentee was denied reliance on post-published evidence to support plausibility.

Clearly, the new referral (G 2/21) will require the EBA to consider the interplay of plausibility and inventive step. It may also take this opportunity to consider the legal basis for the plausibility requirement. The referring decision of the Board of Appeal makes reference to a quote from Sir Robin Jacob on plausibility which suggests that the requirement is based on an interpretation of the European Patent Convention that is not actually supported by the words of the Convention. Is the desire to prevent speculative “armchair” inventions strong enough to overcome this lack of reference to plausibility in the law? We will be keeping a close eye on this referral.


1. T 1329/04 (Factor-9/Johns Hopkins), emphasis added
2. Actavis v Eli Lilly [2015] EWHC 3294, emphasis added