 To account for this difference in the way in which the same object is perceptually represented Peacocke argues that there is an additional layer of nonconceptual content that he calls proto-propositional content, which involves objects, properties, and relations rather than concepts (Peacocke 1992). This level of content accounts for the way in which the square is perceived. When it is perceived as a square one perceives a symmetry about the bisectors of the sides, and when it is perceived as a diamond one perceives a symmetry about the bisectors of its angles. The content is nonconceptual since the observer need not have the concepts of symmetry, or angle, or side, that are mentioned in specifying the content of his perception.