John Cook Wilson, Wykeham Professor of Logic at Oxford from 1899 to 1915, swung British philosophical opinion away from Idealism and towards Realism. Cook Wilson used careful attention to the grammatical form of language - including verbal stress and context - as the method of investigating the foundations of human knowledge. The result was a neo-Aristotelian notion of substances that are directly known to us in perceptual experience. He explained the difficulties of perceptual illusion by appealing to Locke's distinction between primary and secondary qualities. Although he published little in his lifetime, Cook Wilson's teaching had a huge influence on generations of Oxford students. His more famous pupils included Gilbert Ryle and J.L. Austin.