Knowing the limits of what you can prove—a demonstration of self-awareness exploring William James and his Principles of Psychology.

In 1878 Henry Holt, the head of the publishing house Henry Holt and Company, made an offer to William James to write a text book on Psychology for Holt’s American Science series.  It took James twelve years to write his book The Principles of Psychology, which finally was published in 1890. 

It “quickly became an international bestseller, at least to the extant that a two-volume, 1,400 page, scholarly book could become a bestseller.  (After two initial printings of 1,800 copies, there were three more printings by 1899*.”  Shortly after James finished that book Holt requested James write a one volume abbreviated version as another more accessible textbook.  In 1892 Psychology: The Briefer Course was published.  That book went through six printings by 1900.

*: pg vii-viii from David E. Leary’s The Routledge Guidebook to James’s Principles of Psychology, 2018, Routledge.

William James was a wise thinker and demonstrated self-awareness by stating the limits of what he could present as verifiable in his books concerning the relationship of the body to the mind and the mind to the body. 
 
William James writes in his Principles of Psychology:
Pg. vi
Psychology, the science of finite individual minds, assumes as its data (1) thoughts and feelings, and (2) a physical world in time and space with which they coexist and which (3) they know.  Of course these data themselves are discussable; but the discussion of them (as of other elements) is called metaphysics and falls outside the province of this book.  This book, assuming that thoughts and feelings exist and are vehicles of knowledge, thereupon contends that psychology when she has ascertained the empirical correlations of the various sorts of thought or feeling with definite conditions of the brain, can go no farther—can go no farther, that is, as a natural science.  If she goes farther she becomes metaphysical.  …This book consequently rejects both the associationist* and the spiritualist** theories; and in this strictly positivistic point of view consists the only feature of it for which I feel tempted to claim originality.

* Associationism is the idea that mental processes operate by the association of one mental state with its successor states. It holds that all mental processes are made up of discrete psychological elements and their combinations, which are believed to be made up of sensations or simple feelings.  Members of the Associationist School, including John Locke, David Hume, David Hartley, Joseph Priestley, James Mill, John Stuart Mill, and Alexander Bain asserted that the principle applied to all or most mental processes.
** Spiritualist theorizes that mental phenomenon require an additional entity beyond the mind or the brain such as a Soul, or Transcendental Ego, or God.

Pg 4
Bodily experiences, therefore, and more particularly brain-experiences, must take a place amongst those conditions of the mentallife of which Psychology need take account. 

Pg. 5
Our first conclusion, then, is that a certain amount of brain-physiology must be presupposed or included in Psychology.
…it will be safe to lay down the general law that no mental modification ever occurs which is not accompanied or followed by a bodily change.

From David E. Leary’s The Routledge Guidebook to James’s Principles of Psychology, 2018, Routledge.
Pg. 44
Despite his use of ‘accompanied’ and ‘followed’ rather than ‘caused’, however, it must be admitted that James muddied the theoretical waters not only by slipping occasionally into interactionist language, but also, more radically, by seeming to move beyond mind-body dualism, as when he made bodily feelings a constitutive aspect of consciousness and personal identity (in Chs. 9 and 10) and portrayed the emotions in a way that virtually dissolved the separation of body and mind (in Ch. 25).  In these and other instances, he was clearly foreshadowing developments in his later metaphysics (as presented in the first two essays in Essays in Radical Empiricism).

The third and final assumption that provided the basis for James’s psychology was that mental states can and do know at least some physical states.  Not only are mental and physical states related; some of their relations are cognitive.  (Some are also emotional—emotions being for James a way of knowing by personal acquaintance, as the mind ‘inwardly welcomes or rejects’ its objects, as he put it in Principles of Psychology [in chapter VIII: The Relations of Minds to Other Things].
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How the body gives rise to the workings of the mind is still a fascinating topic and books are continually being published to accomplish what James knew he could not do at his time.

  Examples of some excellent books on this topic are:

Alva Noe’s Out of Our Heads: Why You Are Not Your Brain, and Other Lessons from the Biology of Consciousness, 2009, Hill and Wang.

Antonio Damasio’s Self Comes to Mind: Constructing the Conscious Brain, 2010, Pantheon Books.

D. T. Galbraith’s How Consciousness Came Into Being, 2012, Duddingston Books.
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Stanislas Dehaene’s Consciousness and the Brain: Deciphering How the Brain Codes Our Thoughts, 2014, Viking Penguin.