Explain why passivity phenomena are described as the clearest and most severe expression of a disturbance of the boundaries of the self.

Disturbance of the boundaries of the self encompasses passivity phenomena as its clearest and most severe expression, in which the loss of the self-world boundary is expressed as a specific, examinable delusional experience of external control. Passivity phenomena represent this underlying breakdown taken to its most extreme and clinically unmistakable form: rather than a vague sense that the boundary between self and world has weakened, the person develops a specific, fixed delusional conviction that a particular thought, feeling, or action is being directly controlled or inserted by an outside force. This matters clinically because passivity phenomena, being specific and directly examinable, provide the clearest and most reliable evidence of this underlying disturbance, making them a particularly important sign to elicit carefully, since the milder expressions of the same underlying breakdown, such as body image disturbance, can be far more subtle and easily overlooked.