Russell devotes his attention to dismantling arguments for idealism. He explains that different philosophers have varying versions of the same principle: that physical objects exist within the mind. Russell cautions against immediately dismissing idealism and recommends following the logic of previous arguments within the philosophical field. Physical objects produce sense data in the mind, but this does not speak to the true nature of the object. The sensations of the external world vary from person to person. Therefore, sense data can only correspond to the physical object.

Philosopher George Berkeley used logical arguments to prove that physical objects are entirely mental. Russell explains that these arguments have some validity, but they do not follow the thread of logic all the way to the end. Berkeley proposed that sense data is the only existence that humans can be assured of, existing entirely within the mind. When humans cease interacting with the physical object, it does not disappear. This is because it exists within the mind of God. Therefore, the external world is made up of the interaction between human ideas and the ideas of God.