Last December, smiling wryly, a friend asked, “What is a hallucination?” Judging by the way in which the query was posed–the inflections as well as the nonverbal cues–it was clear he had already settled the question in his mind, probably years prior, and was asking to satisfy his curiosity of my opinion on the matter more so than to address a genuine lack of his own. Less apparent, of course, was a precise description of his extant view, but the automatic process by which impressions are formed proceeded undaunted and I concluded that his opinion was aligned with the following: “Who is to say that everything isn’t a hallucination?”

The original question, so simple in construction, implored an answer similarly neat. Somewhat embarrassingly, I did not have a ready reply. My first attempt was to define a hallucination as that which is perceived by one person and not perceived by another who occupies the same environment. On the surface, it seems a sensible definition, but, as so often happens, on closer inspection it proves to be a trip down the rabbit hole. Consider, for example, two people who stand together inside a cube-shaped room. Suppose one person perceives the room as empty save for a 5 foot diameter white sphere hovering in its center, whereas the other person perceives the room as utterly empty. The given definition fails to specify whether the hallucination is the white sphere or the absence of it. Additionally, the definition is ill-suited to account for the multitude of minute differences in perception that exist between any two people, its applicability restricted instead to the absence or presence of singular, distinct perceptions. Over the subsequent weeks, I revisited the puzzle intermittently.

The answer I’ve settled on achieves the desired brevity at the expense of requiring the abandonment of the traditional concept of a hallucination, per se, and the adoption of what I term the Hallucination Factor. The question is thus reframed as “What is a Hallucination Factor?”

A Hallucination Factor is a measure of overlap, ranging from 0 to 1, between the sensuous perceptions of two individuals, where a value of 0 indicates no shared perception and a value of 1 indicates exact correspondence. Between any two people, then, there exists a Hallucination Factor, and the concept could arguably be extended to any two living organisms.

There are a few things to note about the given definition of an HF. First, just as with division by zero in mathematics, a Hallucination Factor with respect to a single individual is undefined. Second, no aspect of the HF attempts to characterize an individual’s perceptions as either real or imagined. Finally, the numerical value of an HF suggests it can be objectively measured. Such measurement is conceivable, provided that what is meant by “measure of overlap” is suitably defined and brain imaging tools are used, but in general, I wouldn’t presume the HF concept to extend beyond subjective use, where, despite the probable inaccuracy of its values, there would still exist valid meaning in the values relative to each other. For example, the HF between Sam and Sally, who are both sober, could be supposed *inaccurately* to be 0.8, and the HF between Sam and Spencer, who is tripping on acid, could inaccurately be supposed to be 0.4, but the relation of the former being higher than the latter would be correct.