Art Critic Martin Gayford recalls a series of his dialogues with David Hockney, who has been described as the world's most popular living painter, exploring painting, landscape, space, time and perception. The critic reveals how, in Hockney's words, drawing makes one 'see clearer, and clearer and clearer still' and Hockney emerges as an incisive and original thinker as well as an exceptional artist.