Chapter 3 introduces Scottish explorer John Ross, who, in 1818, was tasked by the British Admiralty to locate the Northwest Passage, a water route across or around North America, by way of the Canadian Arctic. Upon reaching Baffin Bay, Ross sought to explore Lancaster Sound, the other sounds being impassable. Ross soon discovered that Lancaster Sound ended in a vast expanse of mountains and was thus also impassable; however, his second in command helped determine that the mountains were, in fact, a mirage.

This story illustrates how our senses can fail us. This issue plagued early philosophers, who wondered how we can trust our knowledge if we cannot trust our senses. Protagoras, leader of the Sophist philosophers of the fifth century, believed senses to be the source of knowledge but felt the senses could not be wrong. This viewpoint is known as radical relativism, whereby there is no objective reality, and instead, our sense perception is reality. Schulz states, however, that although we tend to equate sensing with knowing, our perception represents the true essence of error.

Schulz breaks down the process of sensing into two different operations. The first is sensation, whereby the nervous system responds to information in our environment.