Campbell sees similar story structures and images across cultures and eras, leading him to hypothesize that there is an unconscious connection between all humans. Campbell integrates Jung’s theories of the collective unconscious and archetypes to explain the psychological connection between humans that produces universal symbols and motifs. Jung theorizes that the collective unconscious of humanity is a result of the evolution of the brain. There are instincts, impulses, and behaviors inherent to the brain which produce shared means of understanding. Since humans across time and place have had this organ, and “since the imagination comes out of the biological ground, [the brain] is bound to produce certain themes” that manifest as mythological symbols (49). All humans also go through the same stages of life—birth, childhood, adulthood, and death—which, Campbell argues, produce shared mythological stories of transformation. The energy that animates the brain is a mystery, and Campbell argues that all mythology works to describe and understand this invisible life energy.

Throughout the book, Campbell discusses several archetypal images and motifs with multiple examples of each to prove their universality. A large portion of the discussion centers on the hero’s journey, which exists in some form in every culture.