William James was a doctor, a major American philosopher and early experimenter with mind-altering substances at Harvard. He originally published these observations of N2O as Subjective Effects of Nitrous Oxide in Mind Vol. 7, 1882.
You can find his account reprinted in Laughing Gas by by Michael Sheldin, David Wallechinsky (printed 1973, reprinted 1992).
The following are William James' own words:
Some observations of the effects of nitrous oxide gas-intoxication which I was prompted to make by reading the pamphlet called "The anaesthetic revelation and the gist of philosophy" (Blood, 1874), have made me understand better than ever before both the strength and the weakness of Hegel's philosophy. I strongly urge others to repeat the experiment, which with pure gas is short and harmless enough. The effects will of course vary with the individual, just as they vary in the same individual from time to time; but it is probable that in the former case, as in the latter, a generic resemblance will obtain. With me, as with every other person of whom I have heard, the keynote of the experience is the tremendously exciting sense of an intense metaphysical illumination. Truth lies open to the view in depth beneath depth of almost blinding evidence. The mind sees all logical relations of being with an apparant subtlety and instantaniety to which its normal consciousness offers no parallel; only as sobriety returns, the feeling of insight fades, and one is left staring vacantly at a few disjointed words and phrases, as one stares at a cadaverous-looking snowpeak from which sunset glow has just fled, or at a black cinder left by an extinguished brand.
The immense emotional sense of reconciliation which characterizes the "maudlin" stage of alcoholic drunkenness -- a stage which seems silly to lookers-on, but the subjective rapture of which probably constitutes a chief part of the temptation to the vice -- is well known. The centre and periphery of things seem to come together. The ego and its objects, the meum and tuum, are one. Now this, only a thousandfold enhanced, was the effect upon me of the gas: and its first result was to make peal through me with unutterable power the conviction that Hegelism was true after all, and that the deepest convictions of my intellect hitherto were wrong. Whatever the idea of representation occurred to the mind was seized by the same logical forceps, and served to illustrate the same truth; and that truth was that every opposition, among whatsoever things, vanishes in a higher unity in which it is based; that all contradictions, so-called, are of a common kind; that unbroken continuity is of the essence of being; and that we are literally in the midst of an infinite, to perceive the existence of which is the utmost we can attain. Without the same as a basis, how could strife occur? Strife presupposes something to be striven about; and in this common topic, the same for both parties, the differences merge. From the hardest contradiction to the tenderest diversity of verbiage differences evaporate; yes and no agree at least in being assertions; a denial of a statement is but another mode of stating the same, contradictions can only occur of the same thing -- all opinions are thus synonyms, are synonymous, are the same. But the same phrase by different emphasis is two; and here again diffence and no-difference merge in one.
It is impossible to convey an idea of the torrential character of the identification of opposites as it streams through the mind in this experience. I have sheet after sheet of phrases dictated or written during the intoxication, which to the sober reader seem meaningless drivel, but which at the moment of transcribing were fused in the fire of infinite rationality. God and devil, good and evil, life and death, I and thou, sober and drunk, matter and form, black and white, quality and quantity, shiver of ecstasy and shudder of horror, vomiting and swallowing, inspiration and expiration, fate and reason, great and small, extent and intent, joke and earnest, tragic and comic, and fifty other contrasts figure in these pages in the same monotonous way. The mind saw how each term belonged to its contrast through a knife-edge moment of transition which it effected, and which, perennial and eternal, was the nunc stans of life. The thought of mutual implication of the parts in the bare form of a judgement of opposition as "nothing -- but," "no more -- than," "Only -- if,", etc. produced a perfect delirium of theoretic rapture. And at last, when definite ideas to work on came slowly, the mind went through the mere form of recognizing sameness in identity by contrasting the same word with itself, differently emphasized, or shorn of its initial letter. Let me transcribe a few sentences:
The most coherent and articulate sentence which came was this:
There are no differences but differences of degree between different degrees of difference and no difference.
But now comes the reverse of the medal. What is the principle of unity in all this monotonous rain of instances? Although I did not see it at first, I soon found that it was in each case nothing but the abstract genus of which the conflicting terms were opposite species. In other words, although the flood of ontologic emotion was Hegelian through and through, the ground for it was nothing but the world-old principle that things are the same only so far and no farther than they are the same, or partake of a common nature -- the principle that Hegel most tramples under foot. At the same time the rapture of beholding a process that was infinite, changed (as the nature of the infinitude was realized by the mind) into the sense of a dreadful and ineluctable fate, with whose magnitude every finite effort is incommensurable and in the light of which whatever happens is indifferent. This instantaneous revulsion of mood from rapture to horror is, perhaps, the strongest emotion I have ever experienced. I got it repeatedly when the inhalation was continued long enough to produce incipient nausea; amd I cannot but regard it as the normal and inevitable outcome of the intoxication, if sufficiently prolonged. A pessimistic fatalism, depth within depth of impotence and indifference, reason and silliness united, not in a higher synthesia, but in the fact that whichever you choose it is all one -- this is the upshot of a revelation that began so rosy bright.
Even when the process stops short of this ultimatum, the reader will have noticed from the phrases quoted how often it ends by losing the clue. Something "fades", "escapes"; and the feeling of insight is changed into an intense one of bewilderment, puzzle, confusion, astonishment: I know no more singular sensation than this intense bewilderment, with nothing particular left to be bewildered at save the bewilderment itself. It seems, indeed, a causa sui, or "spirit become its own object."
My conclusion is that the togetherness of things in a common world, the law of sharing, of which I have said so much, may, when perceived, engender a very powerful emotion; that Hegel was so unusually succeptible to this emotion; throughout his life that its gratification became his supreme end, and made him tolerably unscrupulous as to the means he employed; that indifferentism is the true outcome of every view of the world which makes infinity and continuity to be its essence, and that pessimistic or optimistic attitudes pertain to the more accidental subjectivity of the moment; finally, that the identification of contradictories, so far from being the self-developing process which Hegel supposes, is really a self-consuming process, passed from the less to the more abstract, and terminating either in a laugh at the ultimate nothingness, or in a mood of vertiginous amazement at a meaningless infinity.