In chapter 4 [0] it is pointed out how advances in modern external observation can work with internal introspective observation in the study of the perception processes. Today cognitive science has made progress up to the brain with many instruments measuring, scanning, and piercing the brain but research seems to end there. Further progress requires introspection research to study how we build up objects by referring concepts to things.  

4-11 [0] I am in a position to follow the processes taking place in my organism up to those in the brain, even though my assumptions become more and more hypothetical the closer I come to the central processes in the brain. The method of external observation ends with the brain processes, more precisely, it ends with what I would observe if I examine the brain with the help of the instruments and methods of Physics, Chemistry, and so forth. The method of inner observation, or introspection, begins with the sensation and extends to include the construction of things out of the material of sense-data.