"Physical interfaces (and by extension, GUIs) cannot combine at will to create new behavior." 'Physical' doesn't seem to quite capture the distinction he's aft

"Physical interfaces (and by extension, GUIs) cannot combine at will to create new behavior." 'Physical' doesn't seem to quite capture the distinction he's after. He uses a guitar as an example of an interface where you can combine at will, and it's a physical interface.