has a more literal function.

The functions of stimuli are called stimulus functions._func for short. When stimulus function refers to physical properties of the stimulus, such as quantity, colour, shape, etc., they are called nonarbitrary stimulus functions. When a stimulus function refers to non-physical properties of the stimulus, such as value, they are called arbitrary stimulus functions. For example, a one dollar bill. The value of the one dollar bill is an arbitrary stimulus function, but the colour green is a nonarbitrary