(a)if it refers to a fact which could be seen, it must be the evidence of a witness who says he or she saw it;

(b)if it refers to a fact which could be heard, it must be the evidence of a witness who says he or she heard it;

(c)if it refers to a fact which could be perceived by any other sense, or in any other manner, it must be the evidence of a witness who says he or she perceived it by that sense or in that manner;

(d)if it refers to an opinion or to the grounds on which that opinion is held, it must be the evidence of the person who holds that opinion on those grounds,

(e)the opinions of experts expressed in any treatise commonly offered for sale, and the grounds on which those opinions are held, may be proved by the production of those treatises if the author is dead or cannot be found, or has become incapable of giving evidence, or cannot be called as a witness without an amount of delay or expense which the court regards as unreasonable; and

(f)if oral evidence refers to the existence or condition of any material thing other than a document, the court may, if it thinks fit, require the production of that material thing for its inspection.