In Du "Cubisme" Metzinger and Gleizes explicitly related the sense of time to multiple perspective, giving symbolic expression to the notion of ‘duration’ proposed by the philosopher Henri Bergson according to which life is subjectively experienced as a continuum, with the past flowing into the present and the present merging into the future. The Salon Cubists used the faceted treatment of solid and space and effects of multiple viewpoints to convey a physical and psychological sense of the fluidity of consciousness, blurring the distinctions between past, present and future. One of the major theoretical innovations made by the Salon Cubists, independently of Picasso and Braque, was that of simultaneity, drawing to greater or lesser extent on theories of Henri Poincaré, Ernst Mach, Charles Henry, Maurice Princet, and Henri Bergson. With simultaneity, the concept of separate spatial and temporal dimensions was comprehensively challenged. Linear perspective developed during the Renaissance was vacated. The subject matter was no longer considered from a specific point of view at a moment in time, but built following a selection of successive viewpoints, i.e., as if viewed simultaneously from numerous angles (and in multiple dimensions) with the eye free to roam from one to the other.