This introductory chapter examines the philosophical problem of hallucination, from its historical roots to its relevance for artificial intelligence. Starting with the distinction between hallucinations, illusions, and dreams, it presents classical skeptical scenarios—Descartes’ evil demon and Putnam’s brain-in-a-vat—that question our trust in perception and our knowledge of other minds. If we cannot be certain that other beings possess genuine mental states, how do we evaluate artificial systems that mimic human cognition? This skeptical tradition provides the framework for a contemporary puzzle: artificial intelligence that produces human-like outputs while also hallucinating, raising fundamental questions about minds, machines, and the boundary between them.