Phaedo, perhaps more than any other platonic dialogue, deals with philosophy as a preparation for the afterlife. The dramatization of Socrates’ death is used as narrative device to discuss the immortality of soul and to set out the theory of Forms (as Adamson rightly noted, this is the first dialogue to set out the well-known doctrine). Without the theory of Forms, in fact, most of arguments in favor of the immortality of soul would be undermined (“reminiscence argument” and “affinity argument”) and the soul would lose his “external” cause.
Socrates against Pre-Socratic materialism
The passages between 96a and 100b are crucial to understand why the theory of Forms is introduced, its relations to the soul and the “second sailing” as a sort of new “method” in reaction to the Pre-Socratic “materialism”.
First of all, Socrates claim that when he was young he was “eager of investigating the nature” (Phædo 96a). Here “φύσις” must be understood as “physical world” of natural phenomena. And the kind of investigations Socrates is referring to as typical of what we call “Pre-Socratic” natural philosophers. This passage is pretty significant from the point of view of the different biographical accounts we have of Socrates: the way Plato is representing Socrates here, in fact, sounds almost as a confession of Socrates himself who admits to have (paradoxically) been a “pre-Socratic” philosopher in his youth, probably a meteorologist with “the head in the clouds”, as Aristophanes depicted him in Clouds.
However, after a couple of lines, Socrates does express repentance for what he was and he believed in. He also expresses his disappointment for the pre-Socratic’s “method” of direct observation of nature and for Anaxagoras’ belief that our νοῦς (mind) arrange and cause all things:
My glorious hope was quickly snatched away from me. [Anaxagoras, in fact] did not assign any real causes for the ordering of things but mentioned as causes air and either and water (Phædo 98c)
To consider “air” and “either” and “water” as real causes is for the Platonic Socrates as to say that the cause of Socrates sitting somewhere is his “bones and sinews” (Phædo 98c). This view, according to Socrates, does not allow “the power which causes things to be placed as it is best for them to be placed” (Phædo 99c). In other words, it does not allow us to really grasp reality, but just “shadows” of it: “[the pre-Socratics] give no thought to the good, which must embrace and hold together all things” (Phædo 99c). Therefore, in order to avoid such a big mistake, the “first sailing” must be abandoned and a new way must be taken: Socrates’ second sailing (or “voyage”) is presented as the only alternative to access the truth that is to say that the truth consists of the knowledge of the Forms and that only the Forms (immaterial, external and eternal) can “cause” things in physical world.
The second sailing
Socrates is aware that approaching the truth through senses, as those who believe air and water to be the real causes of things seem to do, is misleading: in return we will get only “shadows” of reality and this is because our senses are unable to face reality as well as our soul would be blind if we tried to look at the sun directly with our eyes (Phædo 99d-99e). The explanatory purpose of this metaphor is not only to reject empiricism and to strike the inadequacy of our senses for knowledge, but also, perhaps more importantly, to stress the fact that rejecting “sensitive knowledge” is not enough as it is suggested that the soul itself needs to undergo different stages to might be able one day to face the sun (which in Plato usually stands for the Form of Good). Therefore, Socrates seems to come to a startling conclusion: there is no “shortcut” to knowledge. Philosophy (“authentic” philosophy as opposed to “superficial” observation of the physical world) is not an easy task, but rather a voyage that must begin with the study of reality by means of λόγοις (100a). Since the real philosopher is aware of the impossibility of facing truth directly, she does “take refuge” in the λόγοις (“words”). How did Socrates come to this conclusion? Through a hypothetical method consisting of a) assuming something and b) deducing consequences. What is assumed by Socrates is the reality of Forms:
- a) “My point of departure [is] that there are such things as absolute beauty and good and greatness and the like” (100b).
It follows that:
- b) “if anything is beautiful besides absolute beauty is beautiful because it partakes of absolute beauty” (100b).
Because of the introduction of Forms not only knowledge can be explained, but also causation. In this respect the theory of Forms is both an epistemological and a metaphysical theory: the immortality of soul is granted as a necessary consequence of the immortality and eternity of the “Form of soul” which “causes” our soul to be what it is.
It should be noted, however, that Socrates does not assure Cebes that the “second sailing” is necessarily more successful than the method of those who look at the facts of daily life and this is probably a typical Socratic touch: he does not want to reduce the second sailing to a dogmatic doctrine.
 Adamson, Peter (2014). Classical Philosophy: A History of Philosophy Without Any Gaps, Oxford: Oxford University Press, p. 137.
 Ibid., p. 138.