Plato …

• Human well-being (eudaimonia) is the highest human aim.
• As Socrates (Plato's teacher) said in the Meno, knowledge is often remembering.
• In Phaedo, Plato says people are never exactly the same height. We see that things in ordinary life are never actually equal. But since we know what inequality is, we must know what equality is, even though we do not see it physically around us.
• Besides equality, Plato believed that there are other abstract concepts where we see the imperfect in the physical world, but somehow know the perfect. He said we know of things like truth, goodness, (1 + 1 = 2), cubes, circles, and beauty in addition to equality. Things of this sort are the Platonic Forms. They exist independently of the sensible world. Physical objects are imperfect and changeable; they copy the perfect and unchangeable Forms.
• Plato said that since we really know of these supra-sensible realities, and since we cannot learn of them from any physical, bodily experience, we must know of them by remembering them. Our Souls must have known the Forms prior to our births. Plato reasons that therefore our physical bodies must not be necessary for the existence of our souls either before birth or after death; and we are therefore immortal.