How did Hermann von Helmholtz investigate the problem of how the senses report the outside world?

Hermann von Helmholtz investigated how the senses report the outside world by analyzing the physiological mechanisms of sensation and challenging prevailing philosophical assumptions, particularly those of Immanuel Kant and the Nature philosophy movement.1

He addressed the problem by:

• Physiological Analysis: Helmholtz sought to trace sensations through sensory nerves and anatomical structures, such as the human eye and the inner ear, to the brain. He aimed to uncover the complete mechanism of sensation, rejecting the idea that vital forces were responsible for these processes.1
• Refuting Innate Concepts: He specifically challenged Kant’s assertion that concepts like time and space were inherent mental attributes. Through his detailed study of vision, Helmholtz argued that the sense of space is a learned concept rather than an innate one.1
• Addressing Sensory Reliability: He examined the "law of specific nerve energies," which notes that sensory organs report their own sense regardless of the stimulus (e.g., a blow to the eye causing one to "see stars"). By investigating this, he sought to understand how we can have confidence in what our senses report about external reality.1
• Scientific Instrumentation: He developed tools like the ophthalmoscope and resonators to perform precise measurements of sensory function, grounding his philosophical inquiries in empirical data and physics.123