RCW 5.60.050
Who are incompetent.
The following persons shall not be competent to testify:
(1) Those who are of unsound mind, or intoxicated at the time of their production for examination, and
(2) Those who appear incapable of receiving just impressions of the facts, respecting which they are examined, or of relating them truly.
[ 1986 c 195 § 2; Code 1881 § 391; 1877 p 86 § 393; 1869 p 103 § 386; 1863 p 154 § 33; 1854 p 186 § 293; RRS § 1213.]