A sign is an object, quality, event, or entity whose presence or occurrence indicates the probable presence or occurrence of something else. A natural sign bears a causal relation to its object—for instance, thunder is a sign of storm, or medical symptoms a sign of disease. A conventional sign signifies by agreement, as a full stop signifies the end of a sentence; similarly the words and expressions of a language, as well as bodily gestures, can be regarded as signs, expressing particular meanings. The physical objects most commonly referred to as signs (notices, road signs, etc., collectively known as signage) generally inform or instruct using written text, symbols, pictures or a combination of these.
The philosophical study of signs and symbols is called semiotics; this includes the study of semiosis, which is the way in which signs (in the semiotic sense) operate.
Semiotics, epistemology, logic, and philosophy of language are concerned about the nature of signs, what they are and how they signify. The nature of signs and symbols and significations, their definition, elements, and types, is mainly established by Aristotle, Augustine, and Aquinas. According to these classic sources, significance is a relationship between two sorts of things: signs and the kinds of things they signify (intend, express or mean), where one term necessarily causes something else to come to the mind. Distinguishing natural signs and conventional signs, the traditional theory of signs (Augustine) sets the following threefold partition of things:
all sorts of indications, evidences, symptoms, and physical signals, there are signs which are always signs (the entities of the mind as ideas and images, thoughts and feelings, constructs and intentions); and there are signs that have to get their signification (as linguistic entities and cultural symbols). So, while natural signs serve as the source of signification, the human mind is the agency through which signs signify naturally occurring things, such as objects, states, qualities, quantities, events, processes, or relationships.
