A systematic inquiry into the foundations of human knowledge that first examines certainty, arguing for a transcendental science that cannot be reduced to sensation and testing principles such as identity, contradiction, evidence, and consciousness while critiquing modern theories of knowledge. It then turns to sensation, probing the relation between sensations and an external world, the objectivity of spatial and tactile ideas, and whether sight or touch can furnish notions of surface, solidity, and motion. The work combines ontological and psychological perspectives, evaluates criteria like common sense and immediate intelligibility, and outlines methods by which the mind perceives and validates truth.