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The term ‘competent evidence’ is used to refer evidence that is relevant, and of such nature that it can be received by a court of law. It refers to evidence that is appropriate and needed to prove the issue of fact that the parties have made. (definitions.uslegal.com)

It would appear that the RSPB put a recording device on private land without the knowledge or permission of the landowner. The resulting film was then accepted by the sheriff as competent evidence and the gamekeeper was convicted on this evidence. On whose legal authority was the recording device so placed? Were the RSPB acting without (heraldscotland.com)

sound mind. To be mentally calm and self-confident in your actions. In legal literature it means to have the capacity to think, reason and understand oneself. To be competent to perform civil duties. (Urban Dictionary)

A court of cassation is a high instance court that exists in some judicial systems. Courts of cassation do not re-examine the facts of a case, they are only competent for verifying the interpretation of the law. (Wikipedia)