583. [T]here is no reference to any defence of diminished mental responsibility in the Tribunal’s Statute.  The description of diminished mental responsibility as a “special defence” in Rule 67(A)(ii) is insufficient to constitute it as such.  The rule-making powers of the judges are defined by Article 15 of the Tribunal’s Statute, which gives power to the judges to adopt only –

[…] rules of procedure and evidence for the conduct of the pre-trial phase of the proceedings, trials and appeals, the admission of evidence, the protection of victims and witnesses and other appropriate matters.[1]

The Appeals Chamber has held that this power does not permit rules to be adopted which constitute new offences, but only rules of procedure and evidence for the conduct of matters falling within the jurisdiction of the Tribunal.[2]  It follows that there is, therefore, no power to adopt rules which constitute new defences.  If there is a “special defence” of diminished responsibility known to international law, it must be found in the usual sources of international law – in this case, in the absence of reference to such a defence in established customary or conventional law, in the general principles of law recognised by all nations.[3]

9. The Appeals Chamber will address two preliminary arguments. First, the Appellants contend that the two remaining judges in the Butare trial sat in the plenary which amended Rule 15bis on 27 May 2003 and that this impaired their impartiality.  The argument overlooks the fact that judges can in a legislative capacity make rules without prejudice to their right to pronounce in a judicial capacity on the vires or operation of the rules so made. In this case, article 14 of the Statute, which gives a rule-making competence to the judges of the Tribunal, does not prevent them from later deciding in their judicial capacity on the vires or operation of the rules adopted.