Taking as a basis the work of Magoun and others on the 'activating system' of the brain stem, Linn speculates on certain psychological aspects of perception, on barriers to perception, and on the nature of traumatic and therapeutic mechanisms that modify these barriers. He foresees, in the growth of such knowledge, a fulfilment of Freud's hope that direct means may be found for influencing the quantities and distribution of energy in the mental apparatus.