Paul, I've seen that you and kitty have asked this question three times and no one has answered it. So, I'll give it a go. We're told to use "ser" for permanent things and "estar" for temporary things, which is a good rule of thumb, but has exceptions. Under that "rule", look at "witness" as being permanent. Once you've witnessed something, it can't be undone. You've witnessed it. Because of your question, though, I went to an old Spanish textbook. It said "ser" is used to link the subject to a noun, a pronoun, or an adj used as a noun. So, in "Yo soy un testigo" "soy" is linking the subject "yo" to the noun "testigo". ESTAR: "For how you feel and where you are, always use the verb estar." For condition and location use estar. "Yo estoy enfermo y estoy al hospital." SER and ESTAR with adjectives: When you think of a characteristic as being normal or "characteristic" (permanent) of the subject, use "ser". "Paul es intelligente." When the characteristic appears to be the case at a particular time, use "estar". "Estoy muy bonita hoy." Oh, and being dead is evidently a condition (and "dead" is not a noun, either), because ESTAR is used with that, and that's about as permanent as anyone can get.