Final answer:
Plato believed that humans have immortal souls because he thought the soul could exist apart from the body and survive the death of the body. The Theory of the Forms states that the soul participates in eternal forms, giving it an immortal nature.
Step-by-step explanation:
According to Plato, the Forms prove that humans have immortal souls because he believed that the soul could exist apart from the body and would exist after the death of the body. He argued that the soul was more important and immortal than the body. Plato's Theory of the Forms states that every particular thing that exists participates in an immaterial form or essence that gives it its identity. The soul, being immaterial, is able to participate in these eternal forms and therefore has an immortal nature.