Since graphic design is a symbolic exercise, that also makes it a symbiotic exercise. It is a uniquely two-sided design discipline, in that “reaction” is an inherent qualifier for success. Reaction (in this case) is the subjective response by a viewer either towards or against a specific message. Whereas product design or architecture or fashion design or anything in that realm have the benefit of tactile satisfaction, or testability in the sense that you can find out if “this thing works” by allowing users to interact with it directly, graphic design deals with the intangible and emotional only.
The way graphic design “works “ is by provoking a change of perception. An altered thought.