Question

Mr. Johnson is a witness in a criminal trial. During cross-examination, the defense counsel raises an objection to the competency of Mr. Johnson as a witness based on his mental capacity. The defense counsel argues that Mr. Johnson has been diagnosed with a mental illness that affects his ability to perceive and recall events accurately. The prosecution contends that Mr. Johnson's mental condition does not disqualify him from testifying.

Based on the given scenario, discuss the qualifications and disqualifications of witnesses under Sections 21-24 of the Rules of Court. Determine the admissibility of Mr. Johnson's testimony and provide a legal basis for your answer.