The Good-Bad-O-Meter

How come nobody has invented a Good-Bad-O-Meter? It has a trivial UI -- it's a small box, with a needle on the front, and you point it at something and the needle swings towards "good" or "bad". Q, for example, might give one to James Bond, "point this at the girl and only seduce her if it points to 'good'". My particular application is for horticulture; I want something that can tell me if something is a weed or not.

There's some complication, of course, in that things aren't necessarily totally evil (Darth Vader), and some things are neither good nor bad (the Dread Gazebo, Ash from Army of Darkness). But these are just engineering problems.