The Commens Dictionary

Quote from ‘Letters to Mario Calderoni’

…indices, or those signs which represent their objects by virtue of being connected with them in fact, like a clock, or a barometer, a weathercock, a photograph, etc. (The photograph involves a icon, as indeed do very many indices, while on the other hand drawings, portraits in so far they afford information do so because it is known that they actually imitated the natural objects and as such they are indices, not icons. But if you draw a fancy picture of a man with certain physiognomical peculiarities in order to see what sort of an impression of such a man’s disposition you will get in this way, or if you construct a geometrical diagram according to a certain precept and observe certain relations between its parts which appear to be consequences though they were not explicitly required by the precept construction, these things are signs of their objects merely, by virtue of the analogy, and are true icons).