Representations are physical entities that re-present something else. They are the cornerstone not only of any activity that requires information that goes beyond what is immediately presented to agents through some environmental input, but also of the perception of, and interaction with, the environment the way humans and other mammals do. Representations are necessary for perception, for perceptual memory, for planning future actions, and for contemplation, reflection, and conjecture. To represent a thing (in a general sense of “thing”) means to use in the mind or in an external medium, such as a piece of paper or a piece of stone, a sign that stands for the thing that is represented (the representatum). The power that the sign confers to its user is that it can be used as a proxy of the represented thing in the mind of the user even when it is absent.