He was born in London, the son of an architect, and himself trained as one at University College and in Italy. However, his preferences led him simultaneously into a literary career. In 1860 he published The Story of King Arthur. In 1867 he was introduced to Alfred Lord Tennyson, whose house, Aldworth, on Blackdown, he designed; this led to a close friendship, Knowles assisting Tennyson in business matters, and among other things helping to design scenery for The Cup, when Irving produced that play in 1880.
Knowles became intimate with a number of the most interesting men of the day, and in 1869, with Tennyson's cooperation, he founded the Metaphysical Society, the object of which was to attempt some intellectual rapprochement between religion and science by getting the leading representatives of faith and unfaith to meet and exchange views. The society formed the nucleus of the distinguished list of contributors who supported Knowles in his capacity as an editor. In 1870 he became editor of the Contemporary Review, but left it in 1877 and founded the Nineteenth Century (to the title of which, in 1901, were added the words And After). Both periodicals became very influential under him, and formed the type of the new sort of monthly review which came to occupy the place formerly held by the quarterlies. In 1904 he received the honour of knighthood. He died at Brighton.
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