The Commens Dictionary

Quote from ‘Notes on Portions of Hume's "Treatise on Human Nature"’

In their relation to their Dyadic Objects, Signs are, 1^st, those which refer to their objects by virtue of their independent possession of some character of those objects, as a figure of a triangle used in a geometrical demonstration represents any triangle, because it has three rectilinear sides, which it would have, just the same, even if it were not considered as a sign and if there were no other possible triangle in the world for it to represent; 2^nd, those signs which refer to their objects by virtue of being really related to them in existence, as a thermometer is a sign of the temperature of its environment; 3^rd, those signs which refer to their objects not as resembling them, nor as being in fact actually connected with them, but simply by virtue of the circumstance that they will be interpreted as referring to those objects. I term these three kinds, Icon, Index, Symbol.