Section 118 of Evidence Act "Who may testify"
All persons shall be competent to testify unless the Court considers that they are prevented form understanding the question put to them, or from giving rational answers to those questions, by tender years, extreme old age, disease, whether of body or mind, or any other cause of the same kind.
Explanation - A lunatic is not incompetent to testify, unless he is prevented by his Lunacy from understanding the questions put to him and giving rational answers to them.
Section 119 of Evidence Act "Dumb witnesses"
A witness who is unable to speak may give his evidence in any other manner in which he can make it intelligible, as by writing or by signs; but such writing must be written and the signs made in open Court. Evidence so given shall be deemed to be oral evidence
Section 120 of Evidence Act "Parties to civil suit, and their wives or husbands, Husbands or wife of person under criminal trial"
In all civil proceedings the parties to the suit, and the husband or wife of any party to the suit, shall be competent witnesses. In criminal proceedings against any person, the husband or wife of such person , respectively, shall be a competent witness.