At this point, Plato prepares us for a second argument for immortality, based on a doctrine held by Plato but attributed to Socrates here: "learning is no other than recollection." The argument for this position had been laid out in the middle dialogue entitled Meno, in which a slave-boy is induced through Socrates' questioning to prove the Pythagorean theorem, which he had never learned. In the Phaedo, the doctrine is explained in conjunction with the theory of the Forms. Socrates uses equality as an example. Suppose two sensible objects have the same length, that is, that they are equal to each other. The standard for their equality is the Equal itself. That is, they are equal to each other only insofar as they stand in a relation to the Equal itself. They are not the Equal itself because many other things are equal as well. This reasoning applies to other standards such as the Good itself and the Beautiful itself. What makes object equal, good, and beautiful are called "Forms." They do not exist in the objects to which they apply, but rather exist in a realm of their own.Granted that there is knowledge of the Forms, it must be gained in a way other than through bodily experience. Two objects of equal length can be known to be equal only through knowledge of the Equal itself. On the other hand, repeated sense-experience is never sufficient to inform us of the Equal itself. Plato proposes three explanations for our knowledge of the forms. It might have known the forms once, lost the knowledge, and then recollected it. Since knowledge of the forms is not acquired in life, the only other option is that it is possessed at birth. Either we are born with this knowledge or we come to possess it upon being born. But in either case, everyone would have knowledge of the Forms all the time, which is not the case. (John Locke, writing in the seventeenth century, made a similar point regarding what he called "innate ideas." See An Essay Concerning Human Understanding, Book I.) So the only possibility remaining is recollection. Knowledge of the Forms comes only after appropriate training. The training provides a kind of aid to memory that reminds us of what has been forgotten.
Using knowledge of the Equal itself as a paradigm case, we can now construct an argument for immortality based on the assumption that the doctrine of recollection is correct. The core of the argument is simple enough: in order to recall the forms that are forgotten at birth, the soul must have existed before birth.
1. The soul can only know the Equal itself by recollection
2. Recollection requires existence before birth
3. So, the soul existed before birth
4. If the soul existed before birth, then it existed after death [from prior argument]
5. So, the soul exists after death
The central claim of the argument, step 2, appears plausible. But the rest of the argument depends on the establishment of the doctrine of recollection and the previous argument. We have seen that the previous argument is highly dubious. As for the doctrine of recollection, it has been embraced by practically no philosopher after Plato. (It is beyond the scope of these notes to criticize that doctrine here, as we are not reading the main text on the topic.)
