How can a distant round-looking tower illustrate a skeptical argument about perception?

In the context of skepticism, the example of a square tower appearing round from a distance is a classic illustration used to demonstrate the unreliability of sensory experience.1

This argument, famously employed by René Descartes in his Meditations on First Philosophy, serves as a foundational step in his methodic doubt. By observing that our senses can be misled by environmental factors—such as distance or poor illumination—Descartes argues that sensory data cannot be considered an infallible source of knowledge.1

If our senses can deceive us regarding the shape of a simple object like a tower, we cannot be certain that our perceptions of the external world accurately reflect reality. This realization leads the skeptic to reject beliefs derived from sensory experience, pushing the inquiry toward more radical questions about whether we can ever truly distinguish between appearance and objective reality.213