The emotions we have implemented in Feelix correspond to the ones known as ``basic emotions''. The use of this term is still highly controversial among students of human emotions (see [11] for a good account of this controversy), as researchers do not agree neither in the number and subset of emotions that can be considered as basic (classifications range from two to nine), nor in what sense they are so. As a general characterization, we could say that basic emotions seem to be universally found across societies in humans (some authors extend this universal character to other mammals), have particular manifestations associated with them (facial expressions, behavioral tendencies, physiological patterns), have adaptive, survival-related roles that evolved through their value in dealing with situations which are or were fundamental in life, can be seen as prototypical emotional states of a number of emotion families (e.g., rage and anger belong in the same family, anger being the more standard or prototypical case, while rage corresponds to a highly intense anger), and can be taken as building blocks out of which other, more complex emotions form. In our case, the main reason for adopting this hypothesis of a subset of discrete basic emotions is the ease with which their facial expressions are recognized even by children.
Some researchers prefer to characterize emotions in terms of continuous dimensions, rather than as discrete categories. The two most commonly used dimensions (names may vary) are valence (positive/negative) and activation or arousal (calm/excited). These views are not incompatible, and in our model we use a combination of them. In fact, basic emotions can be easily placed in an emotional space defined by these dimensions (see for instance [12]), although two dimensions alone are not enough to distinguish among all the basic emotions--intense fear and anger, for example, are both characterized by negative valence and high arousal. A third dimension, potency (powerfulness/powerlessness), is sometimes added.