“Meanings can be natural, as expressed in the old Scholastic saying Urina est signum sanitatis (‘Urine is a sign of health’); that is, from a person’s urine it is possible to conclude about the person’s health; and this conclusion is immediate, natural and, in most cases, uncontroversial.
In contrast to such a natural sign as urine, language usually is not natural, but conventional: that is, normally there is no immediate, direct connection between a word and what it expresses.
If we had to rely on so-called ‘natural signs’ for our communicative purposes, or communication would be rendered extremely restricted and difficult, if not impossible.”
Jacob L. Mey, “Pragmatics. An Introduction” Oxford, Blackwell Publishers 1993.