A woman describes recurrent, unpleasant thoughts about contamination that intrude against her will, which she finds excessive and irrational, but which she is clear come from her own mind rather than from anywhere else. Before revealing the answer: is this thought alienation?

No. Thought alienation requires the thought to be felt as literally foreign or externally controlled, not merely unwanted. Here, she is explicit that the thoughts come from her own mind, however unwelcome and irrational they feel — this is the defining feature of an obsession rather than thought alienation, which differs precisely in that the obsessional thought is unwanted but still recognised as the person's own. The distinction matters because thought alienation carries different diagnostic weight, historically considered strongly suggestive of schizophrenia, than an obsession does.