Emotion classification, the means by which one may distinguish one emotion from another, is a contested issue in emotion research and in affective science. Researchers have approached the classification of emotions from one of two fundamental viewpoints: Emotion classification, the means by which one may distinguish one emotion from another, is a contested issue in emotion research and in affective science. Researchers have approached the classification of emotions from one of two fundamental viewpoints: In discrete emotion theory, all humans are thought to have an innate set of basic emotions that are cross-culturally recognizable. These basic emotions are described as 'discrete' because they are believed to be distinguishable by an individual’s facial expression and biological processes. Theorists have conducted studies to determine which emotions are basic. A popular example is Paul Ekman and his colleagues' cross-cultural study of 1992, in which they concluded that the six basic emotions are anger, disgust, fear, happiness, sadness, and surprise. Ekman explains that there are particular characteristics attached to each of these emotions, allowing them to be expressed in varying degrees. Each emotion acts as a discrete category rather than an individual emotional state. Humans' subjective experience is that emotions are clearly recognizable in ourselves and others. This apparent ease of recognition has led to the identification of a number of emotions that are said to be basic, and universal among all people. However, a debate among experts has questioned this understanding of what emotions are. There has been recent discussion of the progression on the different views of emotion over the years. On 'basic emotion' accounts, activation of an emotion, such as anger, sadness, or fear, is 'triggered' by the brain's appraisal of a stimulus or event with respect to the perceiver's goals or survival. In particular, the function, expression, and meaning of different emotions are hypothesized to be biologically distinct from one another. A theme common to many basic emotions theories is that there should be functional signatures that distinguish different emotions: we should be able to tell what emotion a person is feeling by looking at his or her brain activity and/or physiology. Furthermore, knowledge of what the person is seeing or the larger context of the eliciting event should not be necessary to deduce what the person is feeling from observing the biological signatures. On 'constructionist' accounts, the emotion a person feels in response to a stimulus or event is 'constructed' from more elemental biological and psychological ingredients. Two hypothesized ingredients are 'core affect' (characterized by, e.g., hedonic valence and physiological arousal) and conceptual knowledge (such as the semantic meaning of the emotion labels themselves, e.g., the word 'anger'). A theme common to many constructionist theories is that different emotions do not have specific locations in the nervous system or distinct physiological signatures, and that context is central to the emotion a person feels because of the accessibility of different concepts afforded by different contexts. Eugene Bann proposed a theory that people transmit their understanding of emotions through the language they use that surrounds mentioned emotion keywords. He posits that the more distinct language is used to express a certain emotion, then the more distinct the perception (including proprioception) of that emotion is, and thus more basic. This allows us to select the dimensions best representing the entire spectrum of emotion. Coincidentally, it was found that Ekman's (1972) basic emotion set, arguably the most frequently used for classifying emotions, is the most semantically distinct. For both theoretical and practical reasons researchers define emotions according to one or more dimensions. Wilhelm Max Wundt, the father of modern psychology, proposed in 1897 that emotions can be described by three dimensions: 'pleasurable versus unpleasurable', 'arousing or subduing' and 'strain or relaxation'. In 1954 Harold Schlosberg named three dimensions of emotion: 'pleasantness–unpleasantness', 'attention–rejection' and 'level of activation'. Dimensional models of emotion attempt to conceptualize human emotions by defining where they lie in two or three dimensions. Most dimensional models incorporate valence and arousal or intensity dimensions. Dimensional models of emotion suggest that a common and interconnected neurophysiological system is responsible for all affective states. These models contrast theories of basic emotion, which propose that different emotions arise from separate neural systems. Several dimensional models of emotion have been developed, though there are just a few that remain as the dominant models currently accepted by most. The two-dimensional models that are most prominent are the circumplex model, the vector model, and the Positive Activation – Negative Activation (PANA) model. The circumplex model of emotion was developed by James Russell. This model suggests that emotions are distributed in a two-dimensional circular space, containing arousal and valence dimensions. Arousal represents the vertical axis and valence represents the horizontal axis, while the center of the circle represents a neutral valence and a medium level of arousal. In this model, emotional states can be represented at any level of valence and arousal, or at a neutral level of one or both of these factors. Circumplex models have been used most commonly to test stimuli of emotion words, emotional facial expressions, and affective states.