Philosophical arguments often include incomplete induction and analogy. It first establishes several known examples with certain logical relationship. All of these examples share the same structure. It then proposes a new argument, unknown whether it is true or not. If this argument has exactly the same structure as the previous examples, it is deduced that the same logic must hold for this argument.
In Phaedo, the Forms of absolute beauty, justice, and goodness are invisible and immortal whereas the so-called beauty, justice, and goodness in the material world are visible and changeable. The body is visible and changeable; the soul is invisible. Following an analogous structure, it is deduced that the soul is immortal.