"In more contemporary accounts of hallucination, it has been difficult to find an unambiguous definition. Nonetheless, it is important to agree on a suitable working definition that will guide theory and research, and in describing efforts at reaching such a definition, we will be able to demarcate hallucinations from other phenomena that might share some phenomenological features. The APA Dictionary of Psychology defined hallucinations as "a false sensory perception that has the compelling sense of reality despite the absence of an external stimulus" (VandenBos, 2007, p. 427; see Exhibit 1.1 for the complete definition). This certainly captures the essence of a hallucinatory experience, although a more precise description should be possible. For example, the statement"despite the absence of an external stimulus" might not be entirely accurate, because some hallucinations are triggered by (irrelevant) external stimuli - for example, patients who start hearing voices when the vacuum cleaner is switched on. Hallucinations have been defined in different ways (see Exhibit 1.1 for a list), although they have a number of elements in common."