Cebes is still not fully persuaded.  Socrates now offers a third argument.[8]  The world is made of simple and complex objects.  Complex object are composed of parts and are likely to disintegrate into those parts or components if they fail.  Simple objects, on the other hand, are more likely to persist precisely because they are simple. 

            The Forms such as the Equal and the Beautiful are simple or “uniform” (monoeidēs) things, apparently by definition.  They seem impervious to change and to decay.  The sensible things that have the property, for instance of being beautiful, are, on the other hand, subject to change and decay. 

            We can distinguish between things that are visible and things that are invisible.  The former seem to be changeable, the latter unchanging.  The body is a visible thing, the soul invisible.  The body is, accordingly, changeable, the soul unchanging. 

            Socrates gathers a list of characteristics to distinguish the two classes.  The body is like the human, mortal, complex, unintelligible, decomposable, and never staying the same as itself; the soul is like the divine, immortal, simple, intelligible, indissoluble, and always the same as itself.  Hence, the soul is like the eternal, the body like what is perishable. 

            Again, Plato offers an attractive argument.  But like the Argument from Opposites, the present argument is an argument from analogy that falls short of being demonstrative.  The soul is like the simple, which tends to be eternal, while the body is like the complex, which tends to be perishable, so probably the soul is eternal, the body perishable.  The argument might give us grounds for hope, but it does not compel us to accept its conclusion. 

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[8] Plato Phaedo 78c-80b.