why view plane normal is negated in lookup function

Hello, I do have a very simple question. The view plane normal ‘n’ is calculated from the ‘eye’ and ‘at’ points as follows, i.e. (at - eye). But why it is negated in lookup matrix.
I am a bit confused. I found in some books ‘n’ is calculated as the opposite, i.e., (eye - at). In this case, in lookup matrix, ‘n’ was not negated. I need some clarification regarding the way of constructing lookup matrix from these two ways ( when ‘n’ negated and when not negated).