James interacted with a wide array of writers and scholars throughout his life, including his godfather Ralph Waldo Emerson, Horace Greeley, William Cullen Bryant, Oliver Wendell Holmes, Jr., Charles Sanders Peirce, Josiah Royce, George Santayana, Ernst Mach, John Dewey, Helen Keller, Mark Twain, James Frazer, Henri Bergson, H. G. Wells, G. K. Chesterton, Sigmund Freud, and Carl Jung.

Early years
William James, with his younger brother Henry James (who became a prominent novelist) and sister Alice James (who is known for her posthumously published diary), received an eclectic trans-Atlantic education, developing fluency in both German and French languages along with a cosmopolitan character. His family made two trips to Europe while he was still a child, setting a pattern that resulted in thirteen more European journeys during his life. His early artistic bent led to an early apprenticeship in the studio of William Morris Hunt in Newport, Rhode Island, but yielded in 1861 to scientific studies at Harvard University's Lawrence Scientific School.

In his early adulthood, James suffered from a variety of physical and mental difficulties, including problems with his eyes, back, stomach, and skin, as well as periods of depression in which he was tempted by the thought of suicide. Two younger brothers, Garth Wilkinson (Wilky) and Robertson (Bob), fought in the Civil War, but the other three siblings (William, Henry, and Alice) all suffered from periods of invalidism.

James switched to medical studies at Harvard Medical School in 1864. He took a break in the spring of 1865 to join Harvard's Louis Agassiz on a scientific expedition up the Amazon River, but aborted his trip after eight months, having suffered bouts of severe seasickness and mild smallpox. His studies were interrupted once again due to illness in April 1867.

He traveled to Germany in search of a cure and remained until November 1868. (During this period he began to publish, with reviews appearing in literary periodicals like the North American Review.) He finally earned his M.D. degree in June 1869, but never practiced medicine. What he called his "soul-sickness" would only be resolved in 1872, after an extended period of philosophical searching.

James's time in Germany proved intellectually fertile, finding his true interests lay not in medicine but in philosophy and psychology. Later, in 1902 he would write: "I originally studied medicine in order to be a physiologist, but I drifted into psychology and philosophy from a sort of fatality. I never had any philosophic instruction, the first lecture on psychology I ever heard being the first I ever gave" (Perry, The Thought and Character of William James, vol. 1, p. 228).

During his Harvard years, James joined in philosophical discussions with Charles Peirce, Oliver Holmes, and Chauncey Wright that evolved into a lively group known as the Metaphysical Club by the early 1870s. Louis Menand speculates that the Club provided a foundation for American intellectual thought for decades to come.

Professional career
James studied medicine, physiology, and biology, and began to teach in those subjects, but was drawn to the scientific study of the human mind at a time when psychology was constituting itself as a science. James's acquaintance with the work of figures like Hermann Helmholtz in Germany and Pierre Janet in France facilitated his introduction of courses in scientific psychology at Harvard University. He established one of the first—he believed it to be the first—laboratories of experimental psychology in the United States in Boylston Hall in 1875. (On the question of this claim to priority, see Gerald E. Myers, William James: His Life and Thought [Yale Univ. Press, 1986], p. 486.)

James spent his entire academic career at Harvard. He was appointed instructor in physiology for the spring 1873 term, instructor in anatomy and physiology in 1873, assistant professor of psychology in 1876, assistant professor of philosophy in 1881, full professor in 1885, endowed chair in psychology in 1889, return to philosophy in 1897, and emeritus professor of philosophy in 1907.

Among James's students at Harvard were such luminaries as George Santayana, W.E.B. DuBois, G. Stanley Hall, Ralph Barton Perry, Gertrude Stein, Horace Kallen, Morris Raphael Cohen, Alain Locke, C. I. Lewis, and Mary Calkins.

Writings
William James wrote voluminously throughout his life; a fairly complete bibliography of his writings by John McDermott is 47 pages long (John J. McDermott, The Writings of William James: A Comprehensive Edition, rev. ed. [Univ. of Chicago Press, 1977 ISBN 0226391884], pp. 812-58). (See below for a list his major writings and additional collections)

He first gained widespread recognition with Psychology: The Briefer Course, an 1892 abridgement of his monumental Principles of Psychology (1890). These works criticized both the English associationist school and the Hegelianism of his day as competing dogmatisms of little explanatory value, and sought to re-conceive of the human mind as inherently purposive and selective.

Epistemology
James defined truth as that which works in the way of belief. "True ideas lead us into useful verbal and conceptual quarters as well as directly up to useful sensible termini. They lead to consistency, stability and flowing human intercourse" but "all true processes must lead to the face of directly verifying sensible experiences somewhere," he wrote.

Pragmatism as a view of the meaning of truth is considered obsolete by many in contemporary philosophy, because the predominant trend of thinking in the years since James' death (1910) has been toward non-epistemic definitions of truth, i.e. definitions that don't make truth dependent upon the warrant of a belief. A contemporary philosopher or logician will often be found explaining that the statement "the book is on the table" is true if and only if the book is on the table.

In What Pragmatism Means, James writes that the central point of his own doctrine of truth is, in brief, that "truth is one species of good, and not, as is usually supposed, a category distinct from good, and coordinate with it. Truth is the name of whatever proves itself to be good in the way of belief, and good, too, for definite, assignable reasons." Richard Rorty claims that James did not mean to give a theory of truth with this statement, and that we should not regard it as such.

James seems to say incompatible things about truth. In addition to truth being what is good in the way of belief, he also says truth is correspondence with reality, or 'the facts'. But this may be interpreted as viewing the property of truth as correspondence with reality while maintaining that the concept of truth is whatever is good in the way of belief. True to pragmatist spirit, he never purported to be providing the necessary and sufficient conditions for truth.

Cash Value
From William James's Pragmatism 1981; from the Introduction by Bruce Kuklick p.xiv.

“James went on to apply the pragmatic method to the epistemological problem of truth. He would seek the meaning of 'true' by examining how the idea functioned in our lives. A belief was true, he said, if in the long run it worked for all of us, and guided us expeditiously through our semihospitable world. James was anxious to uncover what true beliefs amounted to in human life, what their "Cash Value" was, what consequences they led to. A belief was not a mental entity which somehow mysteriously corresponded to an external reality if the belief were true. Beliefs were ways of acting with reference to a precarious environment, and to say they were true was to say they guided us satifactorily in this environment. In this sense the pragmatic theory of truth applied Darwinian ideas in philosophy; it made survival the test of intellectual as well as biological fitness. If what was true was what worked, then scientific investigate religion's claim to truth in the same manner. The enduring quality of religious beliefs throughout recorded history and in all cultures gave indirect support for the view that such beliefs worked. James also argued directly that such beliefs were satisfying—they enabled us to lead fuller, richer lives and were more viable than their alternatives. Religious beliefs were expedient in human existence, just as scientific beliefs were.”

Philosophy of religion
James also did important work in philosophy of religion. In his Gifford Lectures at the University of Edinburgh he provided a wide-ranging account of The Varieties of Religious Experience (1902) and interpreted them according to his pragmatic leanings. Some of the important claims he makes in this regard:

1. Religious genius (experience) should be the primary topic in the study of religion, rather than religious institutions--since institutions are merely the social descendant of genius.
2. The intense, even pathological varieties of experience (religious or otherwise) should be sought by psychologists, because they represent the closest thing to a microscope of the mind--that is, they show us in drastically enlarged form the normal processes of things.
3. In order to usefully interpret the realm of common, shared experience and history, we must each make certain "over-beliefs" in things which, while they cannot be proven on the basis of experience, help us to live fuller and better lives.
4. The investigation of mystical experience was constant throughout the life of James, leading him to experiment with chloral hydrate (1870), amyl nitrite (1875), nitrous oxide (1882), and even peyote (1896). He concluded that while the revelations of the mystic hold true, they hold true only for the mystic; for others, they are certainly ideas to be considered, but can hold no claim to truth without personal experience of such.

Theory of emotion
James is one of the two namesakes of the James-Lange theory of emotion, which he formulated independently of Carl Lange in the 1880s. The theory holds that emotion is the mind's perception of physiological conditions that result from some stimulus. In James' oft-cited example; it is not that we see a bear, fear it, and run. We see a bear and run, consequently we fear the bear. Our mind's perception of the higher adrenaline level, heartbeat, etc., is the emotion.

This way of thinking about emotion has great consequences for the philosophy of aesthetics. Here is a passage from his great work, Principles of Psychology, that spells out those consequences.

“We must immediately insist that aesthetic emotion, pure and simple, the pleasure given us by certain lines and masses, and combinations of colors and sounds, is an absolutely sensational experience, an optical or auricular feeling that is primary, and not due to the repercussion backwards of other sensations elsewhere consecutively aroused. To this simple primary and immediate pleasure in certain pure sensations and harmonious combinations of them, there may, it is true, be added secondary pleasures; and in the practical enjoyment of works of art by the masses of mankind these secondary pleasures play a great part. The more classic one's taste is, however, the less relatively important are the secondary pleasures felt to be, in comparison with those of the primary sensation as it comes in. Classicism and romanticism have their battles over this point. Complex suggestiveness, the awakening of vistas of memory and association, and the stirring of our flesh with picturesque mystery and gloom, make a work of art romantic. The classic taste brands these effects as coarse and tawdry, and prefers the naked beauty of the optical and auditory sensations, unadorned with frippery or foliage. To the romantic mind, on the contrary, the immediate beauty of these sensations seems dry and thin. I am of course not discussing which view is right, but only showing that the discrimination between the primary feeling of beauty, as a pure incoming sensible quality, and the secondary emotions which are grafted thereupon, is one that must be made.”

William James' Bear
From William James' Emotion as given in Joseph E. LeDoux's The Emotional Brain: the Mysterious Underpinnings of Emotional Life:

“Why do we run away if we notice that we are in danger? Because we are afraid of what will happen if we don't. This obvious (and incorrect) answer to a seemingly trivial question has been the central concern of a century-old debate about the nature of our emotions.”

It all began in 1884 when William James published an article titled "What Is an Emotion?" The article appeared in a philosophy journal called Mind, as there were no psychology journals yet. It was important, not because it definitively answered the question it raised, but because of the way in which James phrased his response. He conceived of an emotion in terms of a sequence of events that starts with the occurrence of an arousing stimulus {the sympathetic nervous system or the parasympathetic nervous system}; and ends with a passionate feeling, a conscious emotional experience. A major goal of emotion research is still to elucidate this stimulus-to-feeling sequence—to figure out what processes come between the stimulus and the feeling.

James set out to answer his question by asking another: do we run from a bear because we are afraid or are we afraid because we run? He proposed that the obvious answer, that we run because we are afraid, was wrong, and instead argued that we are afraid because we run:
Our natural way of thinking about... emotions is that the mental perception of some fact excites the mental affection called emotion, and that this latter state of mind gives rise to the bodily expression. My thesis on the contrary is that the bodily changes follow directly the PERCEPTION {of the bear} of the exciting fact, and that our feeling of the same changes as they occur is the emotion {called 'feeling' by Damasio}.

The essence of James' proposal was simple. It was premised on the fact that emotions are often accompanied by bodily responses (racing heart, tight stomach, sweaty palms, tense muscles, and so on; sympathetic nervous system) and that we can sense what is going on inside our body much the same as we can sense what is going on in the outside world. According to James, emotions feel different from other states of mind because they have these bodily responses that give rise to internal sensations, and different emotions feel different from one another because they are accompanied by different bodily responses and sensations.

For example, when we see James' bear, we run away. During this act of escape, the body goes through a physiological upheaval: blood pressure rises, heart rate increases, pupils dilate, palms sweat, muscles contract in certain ways {evolutionary, innate defense mechanisms}. Other kinds of emotional situations will result in different bodily upheavals. In each case, the physiological responses return to the brain in the form of bodily sensations, and the unique pattern of sensory feedback gives each emotion its unique quality.

Fear feels different from anger or love because it has a different physiological signature {the parasympathetic nervous system for love}. The mental aspect of emotion, the feeling, is a slave to its physiology, not vice versa: we do not tremble because we are afraid or cry because we feel sad; we are afraid because we tremble and are sad because we cry.

Philosophy of history
One of the long-standing schisms in the philosophy of history concerns the role of individuals in producing social change.

One faction sees individuals ("heroes" as Thomas Carlyle called them) as the motive power of history, and the broader society as the page on which they write their acts. The other sees society as moving according to holistic principles or laws, and sees individuals as its more-or-less willing pawns. In 1880, James waded into this controversy with "Great Men and Their Environment," an essay published in the Atlantic Monthly. He took Carlyle's side, but without Carlyle's one-sided emphasis on the political/military sphere, upon heroes as the founders or over-throwers of states and empires.

"Rembrandt must teach us to enjoy the struggle of light with darkness," James wrote. "Wagner to enjoy peculiar musical effects; Dickens gives a twist to our sentimentality, Artemus Ward to our humor; Emerson kindles a new moral light within us."