In a great book, “What Is Emotion?”, the author, Jerome Kagan, write on the controversial discussion of human emotions.

“Bertrand Russell once noted that scientists who thought they were studying a simple phenomenon eventually recognized that their original question, say, "Why is the sky blue?" was far more complicated than they had imagined. Although some might regard the queries "What is an emotion?" and "How can we know if a person is experiencing one?" as equally simple, the answers offered are riddled with ambiguity and do not enjoy the more consensual, transparent meanings of such concepts as velocity and heat. Most ancient writers anticipated little controversy when, after consulting intuitions that grew out of personal experiences, they defined an emotion as an appraisal of a change in feeling. Scientists' obligation to find objective signs of their concepts, however, forced them to replace the private psychological states of feeling and appraisal, which resist accurate measurement, with definitions of emotion that relied on taped verbal reports, filmed behaviors, or recordings of biological reactions. This move objectified a subjective state. But because the three sources of evidence often required different conclusions, our understanding of human emotions is more controversial today than it was when Aristotle, Rene Descartes, and Immanuel Kant brooded on this idea.

Although every emotion originates in brain activity, each is first and foremost a psychological phenomenon that is underdetermined by a brain state because each brain profile can give rise to an envelope of emotions. The specific emotion that emerges depends on the setting and always on the person's history and biology. Understanding how history and biology select one state from many contains the same mystery that surrounded the puzzles of why like begets like and why apples fall from trees.

The domains over which psychology is sovereign, like the three regions of Caesar's Gaul, consist of actions, cognitive processes, and bodily feelings, a division that is partially faithful to the brain's organization. However, the theoretically most fruitful concepts for each of these domains and the measurements that award validity to them remain nodes of disagreement. Should motor acts be described in terms of the muscle systems involved, their voluntary or involuntary nature, or their intended goals? Should memory be conceptualized in terms of the information that is registered or the information retrieved? Should feelings be classified in accord with their consequences, origins, brain profiles, or semantic descriptions?

Scientists trying to resolve the frustration of choosing from among alternative conceptions of a phenomenon usually select one of two strategies. One group, for whom Albert Einstein is the unchallenged hero, trusts the mind to detect the initial form of the answers through disciplined thought. These scholars first invent a set of ideas, along with their presumed referents, and then arrange experiments to affirm the validity of their a priori concepts. Physicists committed to the notion of gravity waves and psychologists convinced that the concept of a secure infant attachment is preserved from infancy to adulthood provide two examples. The second group, for whom Charles Darwin is the admired icon, are less trusting of the ambitious products of human imagination and prefer to begin with a reliable phenomenon whose causes, consequences, and contextual constraints can be probed. Only after the relevant evidence has been gathered do these investigators invent concepts to summarize what they have observed. The notions of dark matter in the cosmos and reward centers in the brain illustrate this strategy, which Francis Bacon urged as the only way to challenge the seventeenth century Christian scholars who insisted that theological authority decided what was true.

Put plainly, the choice is between betting on a pretty idea or on a plain but reliable fact before investing energy, time, and money in experimental work. Every intellectual effort, whether in the sciences or in the humanities, balances a tension between a centrifugal force racing toward universal truths and a centripetal one grounded in the details of a concrete observation. Scientists who are certain that they have checkmated nature have learned on too many occasions of an embarrassing exception to their dazzling insight. The discovery that the malformed proteins called prions, which are neither viruses nor bacteria, can be infectious is an example of nature's stubborn reluctance to be understood too completely.

The histories of the natural sciences reveal that the second approach has a better record than the first when a discipline is in an early stage of growth. The first natural philosophers took obvious, reliable facts as the puzzles to understand. Johannes Kepler was able to infer the elliptical orbit of Mars around the sun because he had access to extensive observations gathered by Tycho Brahe and his many observers. Michael Faraday's astute observations allowed James Maxwell to construct elegant mathematical summaries of the observed relations between magnetism and electricity. Darwin's fruitful concept of evolution required the abundant evidence he gathered during the voyage of the HMS Beagle around South America.

These and many other examples illustrate the advantage, when theory is weak and consistent facts are sparse, of attending first to robust phenomena and postponing a premature decision on the correct explanatory concepts. Scientists laboring in less mature fields who have begun their inquiry with a favorite idea have often discovered that their beloved concept was inexact, overly abstract, or, in some cases, had no referent at all. The phlogiston that burning substances gave up to the air and the intelligence Francis Galton thought he was evaluating by measuring sensory acuity and speed of motor response turned out to have no coherent actualizations in nature. Initial explanations in immature disciplines are often flawed because they support ideas that have pleasing connotations. Because Kepler accommodated to the evidence, he was forced to reject the aesthetic assumption that the planets moved with a constant velocity in a circular orbit.

Despite this less than illustrious record for an a priori strategy in young fields, some psychologists and most philosophers seized the bull by its horns and imposed definitions on popular, but ill-defined, concepts before exploring the full range of observations to which the words were supposed to refer. Some examples include consciousness, stress, intelligence, and, the theme of this book, emotion. It will be fruitful to desist from the semantic bickering and begin the more difficult task of figuring out the relations among the six constituents of this concept: provocative events, brain states, detected feelings, appraisals, semantic labels, and actions. Only after this goal has been achieved will we be able to decide how useful the idea of emotion is and how best to define it."