next year and a half was spent in England; but the death of his father (February 1878) and the strain of literary work hastened another collapse, and in April 1878 he again visited the Mediterranean, and afterwards spent some time at the Monte Generoso. In August 1878 he had improved sufficiently to return to England, but another collapse followed at the end of September. As a last chance he was sent to Madeira. The senate of University College recommended that he should retain his chair, and that, if he should recover sufficiently, he should be invited to lecture upon special subjects not involving the strain of regular work. Before the council could act upon this suggestion the end had come. After a brief interval of comparative ease, the case became hopeless, and he died at Madeira 3 March 1879. He was buried in Highgate cemetery. He left a widow and two daughters.
An excellent portrait of Clifford by his intimate friend Mr. John Collier is in possession of Mrs. Clifford. Two portraits after photographs are engraved in the 'Essays and Lectures.'
Clifford's health prevented him from giving more than a fragmentary exposition of views which still needed fuller elaboration. As a philosopher, he was a follower of the English school, and radically opposed to the teaching of modern Hegelians. He venerated Berkeley and Hume, but held that their teaching requires the modification implied in modern theories of evolution. His mathematical genius led him to take a special interest in one doctrine. He thought that Kant's argument, based upon the universality and necessity of geometrical truths, was invincible as against Hume. But he thought that the 'imaginary geometry ' of Lobatschewsky and Riemann supplied the true answer, and showed that even geometrical truths must be regarded as a product of experience. His view is most fully given in his essay on the 'Philosophy of the Pure Sciences.' The metaphysical theory to which he inclined is given in the essays on 'Body and Mind' and the 'Nature of Things in themselves.' He was more inclined than most English psychologists to believe in the possibility of constructing a definite metaphysical system, in which he was probably influenced by his admiration for Spinoza. His doctrine is described by Professor Pollock as an 'idealist monism.' He agreed with Berkeley that mind is the ultimate reality; but held that consciousness as known to us is built up out of simple elements or atoms of 'mind-stuff' − the characteristic phrase which gives the keynote of theories full of suggestion, and showing curious affinities to other philosophies, but not fully worked out. His ethical system, strongly influenced by evolutionist doctrines, was also congenial to his own temperament. He attaches supreme importance to freedom, since all progress implies variation, and the implicit acceptance of formulas is equivalent to death. Here he was also influenced by Mazzini from another side. But in his later work more importance is attached to the 'social factor' and the 'tribal judgment' regarded as an embodiment of the past experience of the race. The second volume of 'Essays and Lectures' contains his application of his leading ideas to ethical and religious questions; especially in the essays upon the 'Scientific Basis of Morals,' 'Right and Wrong,' and 'Cosmic Emotion.' He had contemplated a recasting of his work in a book to be called 'The Creed of Science.' A sketch of the intended contents is given in the 'Essays and Lectures' (i. 71, 72). As he had not the opportunity of completing his design, the essays must be taken only as a collection of fragmentary though luminous suggestions.
As a mathematician, says Professor Karl Pearson, Clifford may be regarded as marking an epoch in the history of this science in England. He was among the first by his writings to raise a protest against the analytical bias of the Cambridge school. Essentially a geometrician he yet regarded geometry as a 'physical science,' whose axioms are the outcome of human experience. So great was his belief in geometry that he even went the length of attempting to explain matter on geometrical principles; an attempt which, however it may be regarded in the future, will at least remain as a witness to future investigators of Clifford's consciousness of the often disregarded truth that matter cannot be explained by mechanism. As a mathematical writer Clifford was marked by a keen power of imagination, rich in its suggestions of new lines of thought and discovery; he was a standing example of the fact that the true man of science, especially the mathematician, is the man of speculation, of tested theory, of keen, albeit disciplined imagination. His 'Canonical Dissection of a Riemann's Surface,' his theory of 'Biquaternions,' and his unfinished memoir 'On the Classification of Loci,' belong to the classics of mathematical literature. As a mathematical teacher Clifford did much (and his influence is still working) to revolutionise the teaching of elementary mathematics; he introduced into England the graphical and geometrical methods of Möbius, Culmann, and other Germans. His uncompleted text-
