Neither in the EPC nor in the case law of the board of appeal are formal rules laid down for the evaluation of evidence. The Enlarged Board of Appeal has recalled that proceedings before the EPO are conducted in accordance with the principle of the free evaluation of evidence (G 1/12, OJ 2014, A114, citing G 3/97, OJ 1999, 245, point 5 of the Reasons; and G 4/97, OJ 1999, 270, point 5 of the Reasons).

Thus the EPO departments have the power to assess whether the alleged facts are sufficiently established on a case-by-case basis. Under the principle of free evaluation of evidence, the respective body takes its decision on the basis of all of the evidence available in the proceedings, and in the light of its conviction arrived at freely on the evaluation whether an alleged fact has occurred or not (see e.g. T 482/89, OJ 1992, 646; T 592/98, T 972/02; see also e.g. T 838/92, in which the board found there was a detailed and consistent body of evidence establishing that a device had been on sale before the patent application was filed).

However, the principle of free evaluation of evidence in EPO proceedings cannot go so far as to justify the refusal of a relevant and appropriate offer of evidence. Free evaluation of evidence means that there are no firm rules according to which certain types of evidence are, or are not, convincing. The deciding body must take all the relevant evidence before deciding whether or not a fact can be regarded as proven (T 474/04, OJ 2006, 129, citing G 3/97, OJ 1999, 245, point 5 of the Reasons). On the other hand, failure to submit evidence despite a board's request to do so may be viewed as a sign that the evidence would perhaps not confirm what has been claimed (see T 428/98).

When evaluating evidence, it is necessary to distinguish between a document which is alleged to be part of the state of the art within the meaning of Art. 54(2) EPC, in the sense that the document itself is alleged to represent an instance of what has been made available to the public before the priority date of the opposed patent, and a document which is not itself part of the state of the art, but which is submitted as evidence of the state of the art or in substantiation of any other allegation of fact relevant to issues of novelty and inventive step (T 1110/03, OJ 2005, 302). In the first situation, a document is direct evidence of the state of the art; its status as state of the art cannot normally be challenged except on authenticity. In the second situation, a document is also evidence albeit indirect; it provides a basis for an inference about, e.g. the state of the art, common general knowledge in the art, issues of interpretation or technical prejudice etc. – an inference which is subject to challenge as to its plausibility. Only a document of the first kind can be disregarded on the sole ground that it is published after the priority date. Documents of the second kind do not stand or fall by their publication date even on issues of novelty and inventive step. Disregarding indirect evidence would deprive the party of a basic legal procedural right generally recognised in the contracting states and enshrined in Art. 117(1) and Art. 113(1) EPC (T 1110/03 cited in T 1797/09 and T 419/12).

Ruling on a refusal to hear witnesses, the board in T 1363/14 held that the principle of free evaluation of the evidence did not apply until after it had been taken and could not be used to justify not taking evidence offered. See also T 2238/15 and all decisions cited there.