Why did rationalists believe some truths could be known through reason alone?

Rationalists in modern philosophy believed that certain truths could be known through reason alone because they viewed the intellect as the primary source of knowledge, capable of grasping truths that transcend sense perception. This perspective is rooted in several key convictions:12

• Logical Structure of Reality: Rationalists maintained that reality possesses an inherently logical structure, making it intelligible to the human mind. They argued that the world is a rationally ordered whole where parts are linked by logical necessity.123
• A Priori Insight: They held that the mind possesses inborn powers of insight—often described as a "natural light"—that allow it to apprehend universals and their relations, such as those found in mathematics and logic. Because these truths are necessary and universal, they are considered more certain than the often-confused data provided by sense experience.23
• Self-Evidence: Thinkers like René Descartes sought to establish philosophical systems based on foundations that were "clear and distinct," aiming for the same rigor found in geometry. They believed that by starting with self-evident principles, one could deduce further truths through reason.4