Socrates now marshals a series of arguments against Simmias’ objection, that soul is a harmony.  First, he points out that the claim that soul is a harmony is inconsistent with the Theory of Recollection, according to which knowledge, at least what we today call a priori knowledge, such as an understanding of mathematics, arises from recollecting principles we had learned before this life.  If so, then the soul exists prior to and independent of the body. 

Harmony, however, presupposes a structure of physical components, such that it could not exist apart from them.  The structure of a lyre is the correct arrangement of pieces of wood and string; without those components, there can be no structure.  Prior to the construction of the instrument, the harmony could not exist.  Accordingly, the Theory of Recollection, which posits a soul existing prior to its body, is incompatible with the Harmony Theory of soul.  This argument is ad hominem, that is, directed towards the individual who holds it, but given that Simmias and Cebes were impressed by the Theory of Recollection, it forces them to choose between competing theories.

            Secondly, harmony seems to be something that can exist more or less: a lyre can be more or less in tune, more or less harmonious.  But a soul seems either to exist or not.  If two things are identical, they will have the same properties.  Two things that are identical may seem to have different properties—Clark Kent is a mild-mannered reporter for the Daily Planet; Superman is able to leap tall building is a single bound—but we may discover that they really have the same properties after all, when Clark Kent retires to a phone booth and emerges as Superman.  In the present case, however, it seems that while harmonies can be either more or less harmonious, souls cannot be more or less souls, so that souls are not harmonies. 

            Thirdly, we hold that the soul rules the body.  There are times, at least, when people exert self-control and resist impulses to eat, or drink, or lash out in anger.  We sometimes resist the body’s urges and make choices that depend, not on present whims or desires, but rational goals or moral principles.  In these cases the soul rules the body rather than vice versa, and this can happen only if the soul is not a dependent arrangement of physical components—if it is not a harmony or epiphenomenon but a command center that can impose its will on the body. 

            Simmias and Cebes accept these arguments readily, and abandon the view that the soul is a harmony.