To speak out thus was one side of his passion for veracity. When it was a matter of demonstrable truth, he refused to be intimidated by great names. Already, in his Croonian lecture of 1858, "On the Theory of the Vertebrate Skull," he had challenged, and by direct morphological investigation overthrown, the theory of Oken, adopted and enlarged upon by Owen, that the adult skull is a modified vertebral column. Again, the great name of Owen, that jealous king of the anatomical world, had in 1857 supported the assertion, so contrary to the investigations of Huxley himself and of other anatomists, that certain anatomical features of the brain are peculiar to the genus Homo, and are a ground for placing that genus separately from all other mammals—in a division, Archencephala, apart from and superior to the rest. Huxley thereupon re-investigated the whole question, and soon satisfied himself that these structures were not peculiar to man, but are common to all the higher and many of the lower apes. This led him to study the whole question of the structural relations of man to the next lower existing forms. Without embarking on controversy, he embodied his conclusions in his teaching.
Thus, in 1860, he was well prepared to follow up Darwin's words in the Origin of Species, "Light will be thrown on the origin of man and his history," and to furnish proofs in the field of Development and Vertebrate Anatomy, which were not among Darwin's many specialities.
When Owen, at the Oxford meeting of the British Association, repeated his former assertions, he publicly took up the challenge. On the technical side, a series of dissections undertaken by himself, Rolleston, and Flower displayed the structures for all to see; on the popular side, Huxley delivered in 1860 a course of public lectures which were the basis of his book, Man's Place in Nature, above mentioned.
Here the principle is actively exemplified: speak out fearlessly at the right moment to strike down that which is demonstrably false. It is the counterpart to the other aspect of veracity which will not say "I believe" to an unverified assertion. These two aspects of the same principle, as has been seen, developed hand in hand in his early career; but it was the active challenge to ill-based authority which, by its courage, not to say audacity, first attracted public notice and public abuse. The other, the apparently negative aspect, came into general notice only after 1869. Its very reserves, however, resting on a statement of reasons for finding the testimony to certain doctrines insufficient, had long provoked assaults from the upholders of these doctrines, which made no less call upon his courage and endurance. As a philosophic position, however, it was not formally and publicly defined until, in the debates of the Metaphysical Society founded in that year, he invented for himself the label of Agnostic. The Society was composed of distinguished men, representing almost every shade of opinion and intellectual occupation; University professors, statesmen, lawyers, bishops and deans, a Cardinal, a poet; men of science and men of letters; Anglicans, Roman Catholics, Unitarians, Positivists, Freethinkers.
The story is told in his article on "Agnosticism," written in 1889 (Collected Essays, v, 237). After describing how it came about that his mind "steadily gravitated towards the conclusions of Hume and Kant," so well stated by the latter as follows:—
The greatest and perhaps the sole use of all philosophy of pure reason is, after all, merely negative, since it serves not as an organon for the enlargement (of knowledge), but as a discipline for its delimitation; and, instead of discovering truth, has only the modest merit of preventing error—
he proceeds:—
When I reached intellectual maturity and began to ask myself whether I was an atheist, a theist, or a pantheist; a materialist or an idealist; a Christian or a freethinker; I found that the more I learned and reflected the less ready was the answer, until, at last, I came to the conclusion that I had neither art nor part with any of these denominations except the last. The one thing in which most of these good people were agreed was the one thing in which I differed from them. They were quite sure they had attained a certain "gnosis"—had, more or less successfully, solved the problem of existence; while I was quite sure I had not, and had a pretty strong conviction that the problem was insoluble. And, with Hume and Kant on my side, I could not think myself presumptuous in holding fast by that opinion….
This was my situation when I had the good fortune to find a place among the members of that remarkable confraternity of antagonists, long since deceased, but of green and pious memory, the Metaphysical Society. Every variety of philosophical and theological opinion was represented there, and expressed itself with entire openness; most of my colleagues were -ists of one sort or another; and, however kind and friendly they might be, I, the man without a rag of a label to cover himself with, could not fail to have some of the uneasy feelings which must have beset the historical fox when, after leaving the trap in which his tail remained, he presented himself to his normally elongated companions. So I took thought, and invented what I conceived to be the appropriate title of "Agnostic." It came into my head as suggestively antithetic to the "Gnostic" of Church history, who professed to know so much about the very things of which I was ignorant; and I took the earliest opportunity of parading it at our Society to show that I, too, had a tail like the other foxes. To my great satisfaction, the term took; and when the Spectator had stood godfather to it, any suspicion in the minds of respectable people that a knowledge of its parentage might have awakened was, of course, completely lulled.
Of his share in the debates the late Prof. Henry Sidgwick gives the following account:—