The uncanny valley--the unnerving nature of humanlike robots--is an intriguing idea, but both its existence and its underlying cause are debated. We propose that humanlike robots are not only unnerving, but are so because their appearance prompts attributions of mind. In particular, we suggest that machines become unnerving when people ascribe to them experience (the capacity to feel and sense), rather than agency (the capacity to act and do). Experiment 1 examined whether a machine's humanlike appearance prompts both ascriptions of experience and feelings of unease. Experiment 2 tested whether a machine capable of experience remains unnerving, even without a humanlike appearance. Experiment 3 investigated whether the perceived lack of experience can also help explain the creepiness of unfeeling humans and philosophical zombies. These experiments demonstrate that feelings of uncanniness are tied to perceptions of experience, and also suggest that experience--but not agency--is seen as fundamental to humans, and fundamentally lacking in machines. (Contains 2 figures.)