That element of which, as we have seen, even the simplest feeling is a complex, I shall call Mind-stuff. A moving molecule of inorganic matter does not possess mind or consciousness; but it possesses a small piece of mind-stuff. When molecules are so combined together as to form the film on the under side of a jelly-fish, the elements of mind-stuff which go along with them are so combined as to form the faint beginnings of Sentience. When the molecules are so combined as to form the brain and nervous system of a vertebrate, the corresponding elements of mind-stuff are so combined as to form some kind of consciousness; that is to say, changes in the complex which take place at the same time get so linked together that the repetition of one implies the repetition of the other. When matter takes the complex form of a living human brain, the corresponding mind-stuff takes the form of a human consciousness, having intelligence and volition.
Briefly put, the conception is that mind is the one ultimate reality; not mind as we know it in the complex forms of conscious feeling and thought, but the simpler elements out of which thought and feeling are built up. The hypothetical ultimate element of mind, or atom of mind-stuff, precisely corresponds to the hypothetical atom of matter, being the ultimate fact of which the material atom is the phenomenon. Matter and the sensible universe are the relations between particular organisms, that is, mind organized into consciousness, and the rest of the world. This leads to results which would in a loose and popular sense be called materialist. But the theory must, as a metaphysical theory, be reckoned on the idealist side. To speak technically, it is an idealist monism.[5]
1877年のクリフォードのエッセイ The Ethics of Belief において彼は証拠が損失したものを信じることは不道徳であると主張した[20]。彼は、ある古くてあまりよい造りではない船に乗客を満載にして海に送る計画を立てた船のオーナーについて述べた。この船のオーナーは船が航海に適していないのではないかという疑問をもっており、この疑問は彼を悩ませ不幸にした（ "These doubts preyed upon his mind, and made him unhappy." ）。船を改装するには多額の費用がかかると考える。すると、やっとオーナーはこれらのメランコリーの反復を克服することに成功した（ "he succeeded in overcoming these melancholy reflections."）。オーナーは軽い心とともに出向する船を見守って...船が海の真ん中で沈み、何も言わなくなったとき、彼は保険金を獲得した（"with a light heart…and he got his insurance money when she went down in mid-ocean and told no tales."）[20]。
クリフォードは、オーナーは船に問題がないと心から信じていても、乗客の死に罪悪感を感じると主張した（ "[H]e had no right to believe on such evidence as was before him."[ii] ）。加えて、彼は船が無事に目的地に到着した場合でも、選択の道徳性は一度決められた時点で永遠に変わらず、盲目的な偶発の実際の結果は重要でないから、結論は依然として不道徳であると説いた。オーナーも少なからず罪があるが、彼の誤った行為は決して見つからない。しかし、当時彼が入手できた情報を考慮すれば、彼に決定権はなかった。
クリフォードはクリフォードの原理（英語版）として知られるように、次のように結論付けた。 "it is wrong always, everywhere, and for anyone, to believe anything upon insufficient evidence."[20]
クリフォードは時空と相対性の完璧な理論を構築する事ができなかったものの、彼が印刷物に残した結果には現代的な概念を予期するものも多い。Elements of Dynamic（英語版）(1878)では、彼は "quasi-harmonic motion in a hyperbola"を導入した。彼の書いた媒介変数表示化された単位双曲線（英語版）は後世の著者が相対論的変数として利用した。彼は別の印刷物で次のように述べている[21]。
The geometry of rotors and motors…forms the basis of the whole modern theory of the relative rest (Static) and the relative motion (Kinematic and Kinetic) of invariable systems.[iii]
1910年ウィリアム・バレット・フランクランド（William Barrett Frankland）は Space-Theory of Matter を平行に関する著作の中で引用した（"The boldness of this speculation is surely unexcelled in the history of thought. Up to the present, however, it presents the appearance of an Icarian flight."[22]）。後年、一般相対性理論がアルベルト・アインシュタインによって進歩すると、多くの学者が、クリフォードはアインシュタインを先取りしていたということを指摘した。例えばヘルマン・ヴァイル （1923）はベルンハルト・リーマンのように、クリフォードは相対性の幾何学的思考を先取りした人物の一人であると言及している[23]。
1940年、エリック・テンプル・ベルは The Development of Mathematics を発表し、この中で、クリフォードの相対性について論じている[24]。
Bolder even than Riemann, Clifford confessed his belief (1870) that matter is only a manifestation of curvature in a space-time manifold. This embryonic divination has been acclaimed as an anticipation of Einstein's (1915–16) relativistic theory of the gravitational field. The actual theory, however, bears but slight resemblance to Clifford's rather detailed creed. As a rule, those mathematical prophets who never descend to particulars make the top scores. Almost anyone can hit the side of a barn at forty yards with a charge of buckshot.
[He] with great ingenuity foresaw in a qualitative fashion that physical matter might be conceived as a curved ripple on a generally flat plane. Many of his ingenious hunches were later realized in Einstein's gravitational theory. Such speculations were automatically premature and could not lead to anything constructive without an intermediate link which demanded the extension of 3-dimensional geometry to the inclusion of time. The theory of curved spaces had to be preceded by the realization that space and time form a single four-dimensional entity.
Riemann, and more specifically Clifford, conjectured that forces and matter might be local irregularities in the curvature of space, and in this they were strikingly prophetic, though for their pains they were dismissed at the time as visionaries.
1990年、ルース・ファーウェル（英語版）とクリストファー・ニー（Christopher Knee）は、クリフォードの先見の承認の記録について調べた[29]。彼らは一般相対性の概念的考えのいくつかを先知したのは、リーマンではなくクリフォードだ（"it was Clifford, not Riemann, who anticipated some of the conceptual ideas of General Relativity." ）と結論付けた。クリフォードの遠謀の認識の欠損を説明するために、彼らはクリフォードが計量幾何学の専門家であったことに注目し、計量幾何学は追求されるには一般的な認識論ではあまりに難解だ（"metric geometry was too challenging to orthodox epistemology to be pursued.）とした[29]。1992年、2人はクリフォードとリーマンの研究を続けた[30]。
[They] hold that once tensors had been used in the theory of general relativity, the framework existed in which a geometrical perspective in physics could be developed and allowed the challenging geometrical conceptions of Riemann and Clifford to be rediscovered.
主な著作
1872. On the aims and instruments of scientific thought, 524–41.
"I…hold that in the physical world nothing else takes place but this variation [of the curvature of space]."
— Mathematical Papers (1882)
"There is no scientific discoverer, no poet, no painter, no musician, who will not tell you that he found ready made his discovery or poem or picture—that it came to him from outside, and that he did not consciously create it from within."
—"Some of the conditions of mental development" (1882), 王立研究所の講義
"It is wrong always, everywhere, and for anyone, to believe anything upon insufficient evidence."
—The Ethics of Belief (1879) [1877]
"If a man, holding a belief which he was taught in childhood or persuaded of afterwards, keeps down and pushes away any doubts which arise about it in his mind, purposely avoids the reading of books and the company of men that call in question or discuss it, and regards as impious those questions which cannot easily be asked without disturbing it—the life of that man is one long sin against mankind."
—The Ethics of Belief (1879) [1877]
"I was not, and was conceived. I loved and did a little work. I am not and grieve not."
^"I believe that, so far as geometry is concerned, we need still another analysis which is distinctly geometrical or linear and which will express situation directly as algebra expresses magnitude directly."
Leibniz, Gottfried. 1976 [1679]. "Letter to Christian Huygens (8 September 1679)." In Philosophical Papers and Letters (2nd ed.). Springer.
^Clifford, William K. 1873. "On the hypotheses which lie at the bases of geometry." Nature 8:14–17, 36–37.
^Clifford, William K. 1882. "Paper #9." P. 55–71 in Mathematical Papers.
^Biggs, Norman L.; Lloyd, Edward Keith; Wilson, Robin James (1976). Graph Theory: 1736-1936. Oxford University Press. p. p. 67. ISBN978-0-19-853916-2
^Clifford, William (1878). “Applications of Grassmann's extensive algebra”. American Journal of Mathematics1 (4): 350–358. doi:10.2307/2369379. JSTOR2369379.
^Clifford, William K. 2007 [1870]. "On the Space-Theory of Matter." P. 71 in Beyond Geometry: Classic Papers from Riemann to Einstein, edited by P. Pesic. Mineola: Dover Publications. Bibcode: 2007bgcp.book...71K .
^Clifford, William K. 1996 [1887]. "Elements of Dynamic" 2. In From Kant to Hilbert: A Source Book in the Foundations of Mathematics, edited by W. B. Ewald. Oxford. Oxford University Press.
Chisholm, M. (2002). Such Silver Currents - The Story of William and Lucy Clifford, 1845-1929. Cambridge, UK: The Lutterworth Press. ISBN978-0-7188-3017-5
Farwell, Ruth; Knee, Christopher (1990). “The End of the Absolute: a nineteenth century contribution to General Relativity”. Studies in History and Philosophy of Science21 (1): 91–121. Bibcode: 1990SHPSA..21...91F. doi:10.1016/0039-3681(90)90016-2.
