The concept of dissociation continues to be vague, confusing, and even controversial, (Dell, 2009, p.225). A principal controversy concerns the notion of defence. In this talk neurophysiological data is presented suggesting that there are physiologically two different kinds of dissociation. The first, and basic form is of disintegration of not only of psychic life but of brain function. It is suggested that certain patients, notably those with the borderline diagnosis, live in a fairly enduring state of ‘primary dissociation’. Intermittently they show ‘secondary dissociation’, a different pattern of brain function with activation of prefrontally connected inhibitory mechanisms. ‘Secondary dissociation’ is conceived as defensive. Data are presented supporting the proposal are derived from studies not only of borderline patients but of PTSD patients who dissociates compared with those who do not. The Jacksonian Model of Self provides an organizing structure for conceiving dissociation as reflected in these data.