The term obsession refers to images, ideas, or words that force themselves into the subject's consciousness against their will, and which momentarily deprive them of the ability to think and sometimes even to act. The term is derived from the Latin obsidere, which means "to sit before," "to lay siege to," and figuratively "to control an audience." From this is derived the noun obsidio, which means "detention," or "captivity," and figuratively "a pressing danger."

Classical psychiatrists had described the experience of a person whose consciousness was besieged by an intrusive thought and who, although lucid and in possession of his faculties, was incapable of stopping it. Philippe Pinel (mania without delusion), Jean-Étienne Esquirol (affective monomania), and Jules Baillarger (madness accompanied by conscious awareness) all distinguished this pathology from mental alienation in the strict sense. But it...