Plato’s doctrine of immortality is taken up in the Phaedo, Republic, and Phaedrus. Plato thought that the human soul survives the death of the body. However, the soul’s memories of its life are washed clean in the River Lethe; the soul then returns as the soul of another person to live a new life from birth. Also in these works, Plato develops his notion of forms, first introducing them in the Phaedo and going on to define them as eternal, changeless, and immaterial. The relationship between real things to the forms is one of participation. A particular cat that might be your pet, for instance, is a cat because it participates with the form of a cat. While your cat might squint or cough up fur balls, the ideal form cat would not be subject to such irregularities. However, not only neutral and beautiful things have their forms, but everything does. That is, bad cat eyesight and fur balls would also have forms in which they participate. In other words, there is the idea or form of a cat that includes all that makes a cat a cat, and then there is the appearance of your particular pet.