 However, a variety of formulations (see below) are possible.Another form of monism, idealism, states that the only existing substance is mental.Although pure idealism, such as that of George Berkeley, is uncommon in contemporary Western philosophy, a more sophisticated variant called panpsychism, according to which mental experience and properties may be at the foundation of physical experience and properties, has been espoused by some philosophers such as Alfred North Whitehead and David Ray Griffin.