The Paris Panthéon is a recurring example in Jean-Paul Sartre’s analysis of mental imagery, used to show that imagining a place is not the same as inspecting it. A mental picture may present the building vividly, but it cannot disclose architectural details that the imaginer does not already know or believe. The example makes the Panthéon a test of the difference between perception, which can teach through observation, and imagination, which presents an object without yielding new information about it.

The Panthéon as a Test of Imagination

Sartre asks whether someone can learn the number of columns supporting the Panthéon’s pediment simply by forming a mental image of the building. The answer is no: the image contains the number the person already believes the Panthéon to have, and may leave the number indeterminate. Unlike looking at the building, imagining it does not permit the viewer to inspect its features and correct an uncertain impression.

This example illustrates Sartre’s distinction between perception and quasi-observation in imagination. A perceived object offers more than is immediately seen, so observation can uncover further properties. An imagined object, by contrast, is presented with all and only the properties attributed to it in the act of imagining. Knowledge—or, more precisely, belief or opinion—comes before the image rather than being discovered through it.

The Columns and the Limits of Detail

The number eighteen makes the distinction concrete. A person may still imagine the Panthéon even if the image does not show eighteen columns beneath the pediment. The image’s identity as an image of the Panthéon does not depend on its accurately rendering that architectural detail; a mistaken belief, or an image too indeterminate to supply a count, can still direct the imagination to the same place.

Alain’s challenge in Système des beaux-arts sharpens the point: anyone who has an image of the Panthéon in mind should try to count the columns on its façade. Sartre extends the challenge to hypnagogic images as well. Even when such a vision seems exceptionally vivid and detailed, its apparent richness does not make it available for methodical inspection. It is given as a whole rather than yielding its features one by one, so the impression of visual abundance should not be mistaken for architectural knowledge.

An Absent Building in the Argument

The Panthéon also appears in a discussion of translating a French term that can mean either filling or fulfilling. Sartre uses the example of visualizing the Panthéon in its absence when he employs a different term meaning to satisfy. In this context, the building is not presented as an object currently observed; it is an absent object toward which an imaginative act is directed.

Across these examples, the Panthéon matters less as a described landmark than as a precise measure of what an image can and cannot do. The imagined building can be recognized and held in mind, but its columns cannot be counted from the image alone. Its role is to expose the gap between picturing a place and acquiring knowledge by looking at it.