Cebes' objection requires a longer answer that includes an account of the nature of reasoned explanation. Certain forms will not admit either of certain pairs of opposites. The characters of the forms are incompatible with the attribution. Socrates employs this method to establish the immortality of the soul. Soul is always associated with life.
Soul excludes death and is imperishable.
Simmias is not completely convinced, but Socrates urges him to repeat and rehearse the argument, according to this method of hypothesis.