The pre-Socratic philosophers—i.e., those who predated Socrates—lived in the early fifth century and sixth centuries B.C.E. Philosophers such as Alcmaeon, Protagoras, Democritus, and Hippocrates introduced concepts remarkably pertinent to modern ideas. Shifting focus from the gods to the natural world, they attributed mental activity to nous (the later spelling of noos), which some even located in the brain. Several of these philosophers believed that our knowledge of the world is only learned through the sense organs. As we can only know what we see, hear, smell or touch, all human knowledge is necessarily subjective and will differ from individual to individual. This belief in the relativism of human knowledge is a radical idea that remains pertinent to modern psychology.