Why do rationalists claim that sense perception can provide uncertain knowledge?

Rationalists argue that sense perception is an unreliable foundation for knowledge because it is frequently misleading and subject to error.1

In the context of rationalism, thinkers like René Descartes utilized methodic doubt to demonstrate the fallibility of the senses. Key reasons for this skepticism include:

• Deceptive Appearances: Sensory experiences can be deceptive, such as when an object appears different from its actual shape or size depending on the distance or perspective of the observer (e.g., a square tower appearing round from afar).1
• The Problem of Dreaming: It is often impossible to distinguish with certainty between being awake and dreaming, meaning that beliefs about the immediate physical world can be based on illusions.1
• Lack of Certainty: Because sensory data can be doubted, rationalists conclude that it cannot serve as the primary source of absolute truth. Instead, they prioritize reason and the intellect, which can grasp innate ideas and logical truths that exist independently of sense perception.213

By relying on the intellect, rationalists seek to establish a foundation of knowledge that is immune to the uncertainties inherent in human experience.21