Mathematician and Philosopher.

William Kingdon Clifford (4 May 1845 – 3 March 1879) was an English mathematician and philosopher. Building on the work of Hermann Grassmann, he introduced what is now termed geometric algebra, a special case of the Clifford algebra named in his honour, with interesting applications in contemporary mathematical physics and geometry. He was the first to suggest that gravitation might be a manifestation of an underlying geometry. In his philosophical writings he coined the expression "mind-stuff".

Source:

1.) "I once wrote a book about the Victorian crisis of faith and entitled it, borrowing from a poem of Hardy's, God's Funeral. I included Carlyle, [...] as well as the out-and-out atheists such as W K Clifford [...]." A N Wilson, 'Browning's faith kept the snake wriggling underfoot', Daily Telegraph, August 20, 2001, Pg. 19.