'Whatever the occasion might be, the pleasures of sight have one characteristic in common - they take you by surprise. They are sudden, swift and unexpected. If one tries to prolong them, recapture them or bring them about willfully their purity and freshness is lost. They are essentially enigmatic and elusive. One can stare at a landscape, for example, which a moment ago seemed vibrant and find it inert and dull - so one cannot say that this lively quality of sight is simply 'out there in nature,' or easily available to be commanded as wished. Nor is it a state of mind which, once acquired, can bend the most stubborn and unrewarding aspect of external reality to its own purposes. It is neither the one nor the other but a perfect balance between the two, between the inner and the outer. This balance is a sort of convergence which releases a particular alchemy, momentarily turning the commonplace into the ravishing.' (Riley, quoted in "The Pleasures of Sight," in Bridget Riley, exh. cat., London, Tate Modern, 2003, p. 214).