To try to catch a thought in its course is like hunting for a blacksnake on a moonless night. The merest whisper of motion alerts its attention, but no sooner have you pounced than your quarry has vanished. Where do our thoughts begin? In what subliminal den do they crouch before emerging, with the softest flit, into our consciousness? And how do we recognize a thought before it slips into the shape of words? Sometimes we don’t verbalize our thoughts, yet they possess the throb of articulation. If I spot a cat lazing on a windowsill, I don’t automatically formulate the thought “The cat is on the windowsill,” but something in my consciousness registers this perception as a latent statement that I could put into words if I wanted to. It is as though underneath every proposition some skeleton of sense lay shimmering, awaiting the flesh of speech.

Certain Medieval philosophers defined God as an “intellect thinking thought” (it works better in Latin: intellectus intelligens intellectum), pure mind in the act of cognition, but this isn’t the way we mortals think. The genesis of our thoughts is wayward, half-formed, slippery. To monitor them requires the attentiveness of a bird-dog or a sniper.

The French poet Paul Valery used to get up every day before dawn to follow the sinuous vagaries of his own consciousness which he then recorded in a series of massive notebooks; these Cahiers constitute perhaps the most alert and detailed register we possess of a powerful mind observing itself observe itself. But isn’t there something hopelessly self-conscious, in a literal sense, about this quixotic practice? For by a kind of cognitive Indeterminancy Principle, Valery’s obsessive dawn-musings must have affected the very direction of his thoughts and threatened to skew his observations at their source. More impressive in my view is the attempt which William James made, around the same time the French poet was beginning to set his alarm clock, in his monumental “The Principles of Psychology” (Dover, in two volumes, 1,408 pages, $16.95 each) first published in 1890. James, though he draws continually on his own personal experience, seeks to give an account that is valid for us all, and not only for geniuses like himself or Valery.

From a scientific viewpoint, of course, much of James’s psychology is antiquated. But his account of human consciousness remains unsurpassed, and the prose in which he conveys this account, is a wonder: supple, edgy, shot through with wit and humor, often lyrical, always unerringly apt. Here, for instance, is how he describes the attempt to track a thought:

The rush of the thought is so headlong that it almost always brings us up at the conclusion before we can arrest it. Or if our purpose is nimble enough and we do arrest it, it ceases forthwith to be itself. As a snowflake caught in the warm hand is no longer a flake but a drop, so, instead of catching the feeling of relation moving to its term, we find we have caught some substantive thing, usually the last word we were pronouncing … and with its function, tendency, and particular meaning in the sentence quite evaporated.

The image of the snowflake is a good example of James’s way with vivid and palpable imagery. He coined the term “stream of consciousness” (or “the stream of thought”) and he struggled to capture the precise undulations of its flow. Like Freud – whose early work he admired and whom he met, together with Jung, in 1909, a year before his own death – James was supremely attentive to those small lapses and fumbles of the mind that are indications of larger concerns. Notice how accurately and sensitively he conjures up the everyday experience of forgetting a name:

Suppose we try to recall a forgotten name. The state of our consciousness is peculiar. There is a gap therein; but no mere gap. It is a gap that is intensely active. A sort of wraith of the name is in it, beckoning us in a given direction, making us at moments tingle with the sense of our closeness, and then letting us sink back without the longed-for term. If wrong names are proposed to us, this singularly definite gap acts immediately so as to negate them. They do not fit into its mould. And the gap of one word does not feel like the gap of another, all empty of content as both might seem necessarily to be when described as gaps.

From this commonplace occurrence, James develops deeper implications. “When I vainly try to recall the name of Spalding, my consciousness is far removed from what it is when I vainly try to recall the name of Bowles.” How can this be? If neither name is present to memory, why does the gap of each feel different? “We can only designate the difference,” James continues, “by borrowing the names of objects not yet in the mind.” We are reminded, as he proceeds, that James was a philosopher as well as a psychologist (the two disciplines weren’t sharply demarcated in his day), for he now draws the wonderful, and properly baffling, conclusion:

But namelessness is compatible with existence. There are innumerable consciousnesses of emptiness, no one of which taken in itself has a name, but all different from each other. … But the feeling of an absence is… other than the absence of a feeling: it is an intense feeling. The rhythm of a lost word may be there without a sound to clothe it; or the evanescent sense of something which is the initial vowel or consonant may mock us fitfully, without growing more distinct.

For most of his life William James suffered a variety of illnesses, some of which were clearly psychosomatic. Certainly he experienced the most intense mental anguish, especially during his terrible crisis of 1872 when, as he described it, “suddenly there fell upon me without any warning, just as if it came out of the darkness, a horrible fear of my own existence. … After this the universe was changed for me altogether.” His father, the redoubtable Henry James Sr., had suffered a similar “vastation.” During his frequent cures and convalescences, William must have pondered the fitful and elusive permutations of consciousness with all the minute attention of a Proust or of his own brother Henry.

Despite his bluff manner, William was what he himself in “The Varieties of Religious Experience” defined as a “sick soul.” (Once, contemplating suicide, he asked himself, “Why not step out into the green darkness?”) In “The Principles of Psychology,” as in all his greatest works, William James’s enduring significance lies not only in his remarkable intellect but in the stubborn and lifelong moral courage it took to transform sickness into insight.