> Icons are direct visual analogs of the things represented.
Counter example: The universal icon for "play", as in to play a movie, is a right facing arrow. It is not a visual analog of anything, since playing a movie is an entirely abstract concept.
The same is true of rewind, pause, and most other controls on a VCR.
Having two icons next to each other, one that looks like a "+" and the other that looks like a "-" indicates a set of controls for increasing and decreasing. Again, these are icons that are not visual analogs of anything; they're entirely abstract.
The fact that a lot of icons are visual analogs is merely for convenience sake. Having a button that looks like an open door helps communicate to people, who've never been taught what that icon means, that this is probably a button that opens a door. That's more convenient than having to educate all users that a completely arbitrary symbol means "open a door".
But, as I mentioned with the VCR control examples, if the thing you're trying to represent doesn't have a visual analog at all, then you just have to design something arbitrary and explain it to people until it becomes common language. Much like a heart symbol.
Counter example: The universal icon for "play", as in to play a movie, is a right facing arrow. It is not a visual analog of anything, since playing a movie is an entirely abstract concept.
The same is true of rewind, pause, and most other controls on a VCR.
Having two icons next to each other, one that looks like a "+" and the other that looks like a "-" indicates a set of controls for increasing and decreasing. Again, these are icons that are not visual analogs of anything; they're entirely abstract.
The fact that a lot of icons are visual analogs is merely for convenience sake. Having a button that looks like an open door helps communicate to people, who've never been taught what that icon means, that this is probably a button that opens a door. That's more convenient than having to educate all users that a completely arbitrary symbol means "open a door".
But, as I mentioned with the VCR control examples, if the thing you're trying to represent doesn't have a visual analog at all, then you just have to design something arbitrary and explain it to people until it becomes common language. Much like a heart symbol.