 The Scottish philosopher Thomas Reid (1710–1796) argues that perception presents us with sensations, which function as signs of objective qualities. Some sensations are artificial signs, instituted by human agreement. Such are words—the connection between ‘cat’ and the animal sitting on my lap being arbitrary. Other sensations are natural signs, and function independently of human agency—for instance, the connection between smoke and fire. Natural signs typically require us to learn their connection to the signified objective quality, as in the case of smoke and fire. However, in some cases a natural sign signifies without requiring prior experience—a child can detect the anger in an angry face and responds to it immediately. Musical expression is explicitly described by Reid as an example of this category: