A patient describes a disturbance affecting the contents of his own mind, attributed to an outside force. Before revealing the answer, decide what distinguishes thought withdrawal from thought insertion.

Ask which direction the disturbance runs: is something being taken away from his mind, or is something foreign being placed into it? Thought withdrawal involves the person's own thoughts being actively removed by an outside force, leaving a sudden gap where a thought had been. Thought insertion involves foreign thoughts being added, experienced as not belonging to the person at all, rather than the person's own thoughts being taken away. Both share the same underlying felt sense of an external agent acting on the mind, but the direction of that action — subtraction versus addition — is what distinguishes them.