According to Platonism, thinking is a spiritual activity in which Platonic forms and their interrelations are discerned and inspected. This activity is understood as a form of silent inner speech in which the soul talks to itself.[24] Platonic forms are seen as universals that exist in a changeless realm different from the sensible world. Examples include the forms of goodness, beauty, unity, and sameness. On this view, the difficulty of thinking consists in being unable to grasp the Platonic forms and to distinguish them as the original from the mere imitations found in the sensory world. This means, for example, distinguishing beauty itself from derivative images of beauty.[23] One problem for this view is to explain how humans can learn and think about Platonic forms belonging to a different realm.[22] Plato himself tries to solve this problem through his theory of recollection, according to which the soul already was in contact with the Platonic forms before and is therefore able to remember what they are like.[23] But this explanation depends on various assumptions usually not accepted in contemporary thought.