Cognitive states and processes are constituted by the emergence, transformation and storage -in the mind, in the brain, or in an artificial system- of the informational structures that are used to stand for a referent. Three basic properties define a mental representation: 1) the information is contained in the state of the system; 2) the representation is used to stand for a referent; 3) the representation preserves the abstract informational structure of the aspect of the world to which it refers. A mental representation can be interpreted as a mental object that has semantic properties. In a wide sense, a mental representation can refer to any mental state with a cognitive content, to any mental states which refers to previous mental states (memory) and to any presentation system that preserves abstract informational structures (such as drawings, symbols or signs).