The concept of representation signifies the unique relation between something which stands for something else and that something else. According to Perner (1993), “[t]he hallmark of representation is that it stands for something else” (1993, p. 2). Here, it is noteworthy to consider that something can be a representation of itself (Crane, 2003, p. 9). For example, consider the following sentence: ‘This sentence is in English’. The sentence means that it is in the English language, which means that it refers to itself. As pointed out by Gottlob Frege, the relation between the representation and the represented is similar to the relation between sense and reference. The notion of sense here corresponds to the representation of something (the referent) in a certain way. A picture, for example, represents its object in a certain way (Perner, 1993, p. 6). Thus, mental representation is a special case of representation. Mental representations also represent the world at large in a certain way. For example, my belief that it’s raining outside means that the grass out there is wet and not dry. Our beliefs, desires and perceptual experiences are examples of mental representations. The representational feature of mental states is well known as intentionality. By ‘intentionality’, we mean the property of mental states to be about, or refer to, something. Mental states have original intentionality. All other representations, like sentences, maps, pictures and words, refer to something, because we human beings interpret them as being representations of something. A picture in itself does not mean anything unless we identify it as, say, the picture of the person whom we can identify as our father or a friend.
Ordinary maps, drawings, paintings, and the words and sentences of natural language are representations that stand for or about something by virtue of how they are deliberately used for usually social purposes by people and perhaps some animals: their status as representations is derived in complex ways from the ideas and intentions of those people and animals. (Georges Rey, 2011, p. 171)
The problem of defining the mental representation is quite old and dates back to Aristotle. According to Aristotle, to represent an object in the world is to have the form of that object in the soul/mind.
The basic idea behind this theory is that to know something is, in a pretty straightforward sense, to be it. You know the red ball when you see it because you have what it has: redness and sphericity. Your mind literally is just what the physical stuff is, because to be red and spherical is just to be inFORMed by redness and sphericity. (Cummins, 1991, p. 4)
In modern philosophy, Berkeley’s uses of the term ‘idea’ can be taken as the equivalent of representation. For Berkeley, and later Hume, a representation of an object is an image which resembles the object. It is like having a picture in the mind which represents the object. This is pointed out by Cummins while explaining the similarity between Berkeley’s position and Aristotelian scholastic metaphysics:
The scholastic metaphysics was gone, but the basic idea was the same: Images represent things in virtue of resembling them—i.e., in virtue of sharing properties with them (though, of course, a sphere in the mind—i.e., as it exists as an image—takes up no physical space, only mental space; it occupies a portion of the visual field, for example) (Cummins, 1991, pp. 5-6).
The most puzzling and controversial issue here is how mental states can represent something. What makes a mental state stand for something else? There are two approaches in the literature: 1. Intentionalist; and 2. Naturalist theories of mental representation. According to the intentionalists, mental states get their representational status due to the intentions of the individual. According to the naturalists, the representational character is defined by (i) the causal relation between the mental state and the object of representation, and (ii) the functional role of the mental state.
The anti-reductionists argue that intentionality, like phenomenal character, is also an irreducible feature of the mental states. In other words, neurophysiological states of the brain cannot have original intentionality. Rey (2011) points out “[d]ualists such as Descartes and Brentano have argued that rationality and intentionality aren’t assimilable to a general physical theory of the world” (2011, p. 172). The representational theory of consciousness, however, is inspired by the discoveries in cognitive science that our brain uses computational and representational processes similar to modern computing machines. Like the program of a computer, our brain also employs a formal representational system which has much in common with our use of natural language and, thus, is called the “language of thought” (Fodor, 1975).
 Following the distinction made by Dalenoort between subjective and objective representations, we can say that mental representations fall in the category of subjective representations. Dalenoort explains the distinction between subjective and objective representation thus: “The objective ones are the models and ways of modelling of science: any science, in describing its subject matter, uses representations, and follows specific rules for its descriptions and reasoning. The subjective ones may be taken, in a broad sense, as our cognitions. Even our unconscious activities may be considered representations, and their roles may be discovered by the influence they may exert on our conscious activities. In cognitive science, and in the philosophy concerned with cognition, it seems that the term representation is always used in the subjective sense; usually the expression internal representation is used” (Dalenoort, 1990, p. 229).
 “Cognitive science speaks openly and unabashedly of inner mental representations, and calculations and other operations performed on these inner representations. The mind, it proclaims, is a computing device, and as Jerry Fodor, a leading ideologue of cognitive science, has put it, ‘no computation without representation’” (Dennett, 1983, p. 214).