Note that symbols are considerably more sophisticated than signs, and that quite a few non-human animals are quite capable of understanding signs, but all non-human animals find it hard or impossible to understand symbols.

Part of the difficulty is that symbols depend on convention, and as a rule have nothing in common with whatever they mean, and another part of the difficulty is that symbols may stand for mere ideas, fantasies, or even impossibilities, and thus require that whoever understands the symbol is aware that it may refer to what someone thinks or feels or what is thought or felt about, or both.