  But for those who already know the term, this does not happen by hearing the word “dogs” then deriving or inferring some object that has some peculiar fitness to the word. The relation of signs to things they signify in spoken and written language is almost always arbitrary in the sense that there is no similarity between dogs and the word “dogs”. In this sense, “dogs” is an artificial or artifactual sign, for Reid, in contrast to natural signs. Blushing signifies one of a small number of mental states because blushing is caused by events of embarrassment, anger, or romantic stimulation. Blood rushing through the subcutaneous circulatory system in the face reliably occurs, across members of our species, in response to a small set of stimuli with apparent universality, making blushing a natural sign. We discover this relationship by observation and experience.