One of the challenges to design is to create artifacts with the smallest learning curve to the user. A good design is one that is self-evident and easy to use (hence the famous Mies van der Rohe less is more).
Design frequently uses metaphors, which are nothing less than the interpolation of the meaning between two different objects, as a tool to accomplish objectives. Think about a pictogram; it is a graphical representation of the minimal characteristics that are needed of to define its meaning; one circle for the head, four sticks and a rectangle will represent of a man. Of course in reality we have never seen something like this, but we recognize it instantly because both the pictogram and our mind share the same metaphor of man. (if you see a man far away on a desert, he will be perceived as being exactly 4 sticks and a circle for head, but of course that’s not what he is)
And while we are more used to metaphors in literature (“It is the east, and Juliet is the sun.” -Shakespeare), visual metaphors work somehow in a different fashion. Visual metaphors are always bi-directional, unlike literal ones. So we cannot say “the sun is Juliet” without losing the meaning, but every visual metaphor will have to be reversible. The man pictogram has to look like a man, and a man has to look like the pictogram that represents him.
Because we have this bi directionality, visual metaphors don’t need to be based on reason as they rely on perception. The creative mind (it takes a creative mind to interpret any kind of image) is mainly oriented by perception, and only in a later stage by reason (Gardner 1993, ref. Barry 1997 pp 72, 73), we can link concepts that are opposites if we find resemblance on the perceptual level. This is one of the reasons why for instance Ferrari (check out the amazing post about it in graphicology) can get away with the code bar representing Marlboro.
It’s purely a perceptual metaphor and fully justifiable as not being connected to the cigarettes brand through a rational point of view, but still it works. Subliminal imagery is a whole “science” that takes advantage of this invisible to the reason kind of metaphor.
A good metaphor is always obtained by stripping the object to its most fundamental qualities. Think about a table: four legs and the top. They don’t represent any table in particular, nor all the universe of tables as there we can find them with 3, 5 or more legs, but it is the simplest universal representation for table. This is nothing less than a metaphor.
Every object, feeling, word or element in the world is represented internally (in our minds) by some sort of metaphor, and the key to design efficiently is to collect the elements that are common to every table, and unify them in a visual representation.