An Argument for the Immorality of the Soul - Excerpt from Plato's "Phaedo"

In Plato's Socratic dialogue The Phaedo, he presents five arguments for the immortality of the soul, i.e. that it will survive the death of the physical body. Here is the third of those arguments taken from paragraphs 77 - 86. This third argument begins with Socrates and Cebes going back and forth in dialogue recognizing a common fear, that at death the soul will break apart like the body and we will cease to exist. Is the soul something that is made of parts like the body? Or is it more perfect? We see here in this argument that Plato's epistemology leads him to the conclusion that the soul is immortal.

-Compounded Versus Simple- and -Physical Versus Intellectual-

Socrates says that it is things that are made up of parts, or "compounded," that by definition are those can also break apart. Now what if something does not have many parts, but is "uncompounded"? Then wouldn't that mean that it could not be broken apart? And if something cannot be broken apart then it would also seem that that thing doesn't change, as things change when their parts are altered. "And the uncompounded may be assumed to be the same and unchanging, where the compound is always changing and never the same..." 

Next Socrates points out that ideas are, by nature, like the uncompounded. Universal essences, like
beauty, justice, quality are things that don't change with time. Whereas those physical things which participate in beauty are always changing and altering their nature. Thus it is clear that the physical things which can be sensed by the body change while those which can only be seen with the intellectual mind don't change. Thus there is a dual nature of things here, the "seen" and the "unseen." 

Which then is the soul more like, the changing body or the unchanging ideas of the mind? Socrates makes it clear that the soul is more truly itself when it participates in the flight from the senses and retreat into the eternal ideas of the mind. 

"But when returning into herself she reflects; then she passes into the realm of purity, and eternity, and immortality, and unchangeableness, which are her kindred, and with them she ever lives, when she is by herself and is not let or hindered; then she ceases from her erring ways, and being in communion with the unchanging is unchanging."

Thus the soul must also be like the ideas in their eternity. The body breaks apart because of its parts and sensible nature, but the soul cannot break apart because it must be the same type of thing as the ideas which it knows. 

"Then reflect, Cebes: is not the conclusion of the whole matter this?-that the soul is in the very likeness of the divine, and immortal, and intelligible, and uniform, and indissoluble, and unchangeable; and the body is in the very likeness of the human, and mortal, and unintelligible, and multiform, and dissoluble, and changeable.  Can this, my dear Cebes, be denied?"

Conclusion

Socrates makes a strong argument for the differing nature of the soul/mind and the body. To put it in a simpler way, we could summarize it like this:

1) The body is something physical.

2) Physical things are composed of parts and can therefore break apart. 

3) Ideas are not physical things. 

4) Ideas are not composed of parts and do not change.

5) The soul is more akin to the intellectual knowledge of ideas. 

6) Therefore, the soul must itself be like those ideas, non physical and simple. 

7) Therefore the soul will survive the death of the body and be eternal.