In post #79 I introduced Plato’s argument for the immortality of the soul in Book X of his dialogue Republic (all quotations will be from Cornford’s translation). I then discussed the argument’s central claim – the essential destructibility claim – which Eric A. Brown formulates as follows:
Essential Destructibility Claim (EDC): If there were no natural evil of y capable of destroying y, then nothing could destroy y (but if there is such a natural evil, then y can be destroyed by something other than the natural evil of y).
Lastly, I addressed some weak objections to the argument. Now I would like to look at some more powerful objections. Here, again, is the argument:
(1) The natural evil of y makes y bad and may destroy y by dissolving it from within (608d-609a).
(2) If there were no natural evil of y capable of destroying y, then nothing could destroy y (609b).
(3) The vice of injustice, and its vicious results like ignorance, cowardice, and intemperance, is the natural evil of the whole soul. (609b).
(4) But injustice doesn’t destroy the soul; it only makes it bad (609c-d). Indeed, the existence of evil fills the soul with an “unsleeping energy” and makes it feel “very much alive” (609d ff.).
(5) Therefore the soul cannot be destroyed: it is immortal (611a).
Bear in mind that no demonstration of the soul or its properties will be uncontroversial. Perhaps the most we can hope for is an argument with a probable conclusion that rests on rationally defensible ideas.
Objection 1: The argument only works if there is one natural evil; but the notion of one natural evil for something isn’t convincing.
One possible response to objection 1: Plato, like Aristotle after him, is a natural virtue ethicist and perhaps an early forerunner to natural law theory. As such, he is committed to the idea, so popular in these approaches to morality, that each thing has a nature with an essential function or set of functions which, when actualized properly, allows for health and flourishing. And this notion of things having natural functions leads him to assert that things also have particular natural evils. This vision of natural function is, as we have seen, the vision that lies behind the whole argument of the Republic: each person has a proper function and should, if justice is to be realized, do their proper thing in their proper place. The functions of the soul, too, have their proper relationship and must be organized in a particular fashion if a soul is to flourish. And reality itself has a “divine order” (500a) both in the “unchanging and harmonious order” of, on the one hand, the Forms and The Good in the intelligible realm and, on the other hand, the proper positions of the planets in the visible realm that provide an imperfect image of this eternal harmony (528e-530c; 615c). Now this is not the place for a full defense of all these ideas. But clearly they are venerable ideas that can be defended. Thus objection 1 is really an objection to the central argument of the Republic and, by extension, much of natural virtue ethics and law theory law theory as well. Plato is not just introducing the EDC out of context or without theoretical backing; he is presenting something that coheres with his philosophical vision as Brown notes:
“In fact, the Republic is consistently, emphatically concerned about internal health and decay, in contrast to external threats. Thus, the Republic does not say that a city is invulnerable to external attacks, but it does say that if that city is just and healthy, it has nothing to fear from those attacks. That is why when the Kallipolis falls, it will fall from within (546a1 ff.). On this background, it is natural to think that anything invulnerable to internal decay would be indestructible. Since these thoughts lie behind the articulation of natural evils, the essential destructibility claim is a plausible inference.” (303)
Anyone who has sympathies with Plato’s overall argument in The Republic and natural virtue and/or law may not find this objection fatal.
Objection 2: Some have argued that Plato’s argument begs the question: it assumes something as true in the premises that needs to be proven in the conclusion. More specifically, it would seem that, in order to claim the soul cannot be destroyed by bodily destruction, Plato has to prove the soul and body are radically different substances. After all, if human beings are just material beings then it seems obvious they can be destroyed by a multitude of material objects. But rather than prove humans have both an immaterial soul and a material body, he just assumes the soul is immaterial and then gets on with the fallacious demonstration. Julia Annas puts the objection well:
“What is to prevent the relation between body and soul being of this kind, destruction of the body carrying with it destruction of the soul? Plato insists vehemently that this is not so (610a-b), but only by assuming what is never argued for, that the soul is a different kind of thing from the body, and distinct from it in being unaffected by what happens in the body….we find a snappy little argument for a controversial conclusion depending on a question-begging premise which is never argued for.” 
One possible response to objection 2: It is true that some of Plato’s writings suggest he is what has become known as a “substance dualist”. A substance dualist is someone who thinks that the mind/soul and body are two radically different kinds of substances that interact with one another but which cannot be reduced to one another. And typically a substance dualist will maintain that our true self is not the body but the soul. Plato seems to maintain both these aspects. He seems to think the soul is the seat of our true self which interacts with the body, uses the body, and can be corrupted by the body in many ways; but it is an immaterial substance not a material one like the body. And this substance has powers matter doesn’t have: for example, it can originate motion, desire, think, feel, have purposes, gain knowledge, feel shame, and enjoy pleasure and pain. Now, it seems true that Plato is assuming this position in his argument. But does it immediately follow that he is begging the question? Brown doesn’t think so: “One could well hold that soul and body are two different sorts of thing and still maintain that the soul is mortal….Plato’s assumption of soul-body dualism is necessary for his argument, but it is hardly sufficient” (311). Substance dualism as such doesn’t entail that the soul is immortal. For example, someone, say an emergent dualist, might believe that a substantial immaterial mind emerges from the material brain in such a way that the destruction of the brain would necessarily entail the destruction of the mind. And Plato, far from claiming that the soul is “unaffected by the body” as Annas says, claims exactly the opposite: the soul can, if it has the wrong orientation (towards the physical pleasures of the body for example), be adversely affected by the body. This point is made in the Republic with reference to the sea monster Glaucus and the countless evils that beset the embodied soul (610c-612a).
Now, this doesn’t mean we should over look Plato’s dualism and its role in the argument. But we should be sensitive to the fact that the dialogical context of the argument is primarily moral rather than metaphysical: Socrates is trying to get Glaucon to take justice seriously in the face of the nothingness of all time. Therefore an extended discussion of metaphysical issues may not be called for. Moreover, we must not forget Socrates refers to “other proofs” for the immortality of the soul after he presents the one he has just given (611c). It is plausible that Plato is dropping a reference to his dialogue Phaedo which, in recounting the last hours of Socrates’ life, goes over four other proofs for the immortality of the soul. One of these arguments, the so-called argument from recollection, would establish substance dualism if sound. Here is a reconstruction of the argument by Tim Connolly:
(1) Things in the world which appear to be equal in measurement are in fact deficient in the equality they possess (74b, d-e).
(2) Therefore, they are not the same as true equality, that is, “the Equal itself” (74c).
(3) When we see the deficiency of the examples of equality, it helps us to think of, or “recollect,” the Equal itself (74c-d).
(4) In order to do this, we must have had some prior knowledge of the Equal itself (74d-e).
(5) Since this knowledge does not come from sense-perception, we must have acquired it before we acquired sense-perception, that is, before we were born (75b ff.).
(6) Therefore, our souls must have existed before we were born. (76d-e) 
If our souls existed before we were born then, to be sure, they are different in kind from our bodies. This difference is brought out in another proof in Phaedo where we see the soul is most akin to the divine in its immateriality and simplicity and thus will probably not split up and die:
(1) Something is liable to be split up into its component parts only if it is composite or made of parts (78c).
(2) The invisible Forms of things remain the same and do not tolerate any change whatsoever (they are divine, deathless, intelligible, uniform, and indissoluble) (78cd).
(3) The many visible particulars of the world never remain the same as themselves or in relation to each other (they are human, mortal, multiform, unintelligible, soluble) (78e).
(4) The soul investigates by itself when it passes into the realm of the invisible Forms (79d).
(5) The soul is dragged down by the body to the things that are never the same when it investigates through the bodily senses (79c).
(6) Like is known by like (unstated premise; an axiom for Plato).
(7) The body is like the many visible things (80b).
(8) The soul is like the invisible Forms (80b).
(9) So, the soul is “altogether indissoluble or nearly so” (80b).
Socrates argues that the soul, if it leads a philosophical life, can make some degree of contact with the realm of Being and the Forms. The soul can make such contact because it is essentially akin to the realm of Being. Socrates says “Soul is most similar to what’s divine and deathless and intelligible and single-formed and indissoluble and always keeps to the self-same condition with itself. Body, in its turn, is most similar to what’s human and death-bound and many-formed and unintelligible and dissoluble never keeps to the self-same condition with itself.” The Forms, the unchanging, immaterial objects we come to know when we know something, are simple: they are not physical composites. The soul, insofar as it can make contact with them, must be akin to them as well: it must also be immaterial. This notion of kinship resonates well with The Republic. For example, Socrates notes, in his discussion of Glaucus, that he thinks the soul is “akin” to the “divine, immortal, and everlasting world” (611e). This notion of kinship also appears when Socrates describes the philosopher as someone who cannot linger among the multiplicity of physical things but rather passionately seeks the essences of things with the “part of his soul which can apprehend reality because of its affinity therewith” (490a-c). And we encounter a similar passage in the Allegory of the Cave where Socrates asserts: “If, freed from these [affinities to the mortal world], the soul were turned round towards true reality, then this same power in these very men would see the truth as keenly as the objects it is turned to now” (519a ff.). So the affinity argument works well along with the argument from indestructibility.
There are other arguments that Plato uses that can establish a form of dualism, such as the Wooden Horse argument in his dialogue Theaetetus. I will cover this argument here. For now the key is to see that Plato need not be understood to be begging the question in the argument from indestructibility, nor is he without resources to establish some form of dualism.
Objection 3: Seth Benardete presented a powerful dilemma that challenges not only the argument from indestructibility but the whole coherent structure of the Republic as well. Consider this passage: “Either the proof requires a premise that contradicts that of the Republic—the soul must be noncomposite—or, contrary to his proof, the vices of the soul are fatal to the soul (611b5-7), and the perfectly ordered soul of the philosopher is alone deathless.”
One possible response to objection 3: After he concludes the argument from indestructibility, Socrates emphasizes that “we must not think of the soul, in her truest nature, as full of diversity and unlikeness and perpetually at variance with herself”. He claims his account of the tripartite soul (reason, spirit, and appetite) in Book IV “could hardly be everlasting” because it is a composite; and he urges Glaucon to see the earlier account as properly reflecting the soul’s present embodiment in, and transaction with, the body. The soul, in its pure state, may be simple. If this is the case we could avoid the second horn of Benardete’s dilemma: we could say that it is possible for everyone to be immortal, even the unjust, because the soul is not composite and thus cannot disintegrate. But can we avoid the first horn of the dilemma? Can we avoid retroactively dismantling the argument of the Republic because we now commit ourselves to a simple rather than complex vision of the soul? What would happen to all the isomorphic analogies that support Socrates’ argument?
Well, Christopher Shields argues that the three parts or functions of the soul Plato introduces in Book IV should be understood as conceptual parts of a simple substance rather than aggregative or organic parts of a complex substance. He distinguishes these three types of parts as follows:
- Aggregative part: x is an aggregative part of y if and only if: (i) x is a portion of y; (ii) x can exist as x after the dissolution of y (for example, a marble in a pile of marbles).
- Organic part: x is an organic part of y iff: (i) x is a portion of y; (ii) x is a functionally defined entity; and (iii) x is parasitic on y for its identity conditions (for example, the president of a university, a door, your heart).
- Conceptual part: x is a conceptual part of y iff: (i) x is portion of y; (ii) x is not a functionally defined entity; and (iii) x is parasitic on y for its identity conditions (for example, the center of Ireland, the terminus of line segment AB, or, to use Plato’s example at 436e, the axis and circumference of a spinning top).
Shields argues that Plato’s effort in Republic Book IV to demonstrate the parts of the soul is only committed to conceptual parts as is shown with his example of the parts of a spinning top. Since such parts are consistent with something simple, there is no need to see an inconsistency between, on the one hand, Socrates claims about the soul’s simplicity and, on the other hand, the soul as having three parts as discussed earlier in Book IV. If this is the case then the first horn of Benardete’s dilemma—that Socrates’ proof requires a premise that contradicts that of the Republic—is thereby avoided as well. Shields’ argument is as follows:
(1) Plato’s argument for soul-division in Republic IV establishes at most conceptual parts.
(2) If (1), that argument is compatible with the form of simplicity required for the Kinship Argument of the Phaedo and the Indestructibility argument of Republic X.
(3) Hence, Plato’s argument for soul-division is compatible with a simple soul.
Objection 4: Recall premise (4) of the Indestructibility argument for the soul: injustice doesn’t destroy the soul; it only makes it bad (609c-d). Now, how is it the case that Socrates knows that wickedness “brings about the death of other people to the best of its power, and, far from being deadly to the wicked man himself, it makes him very much alive and fills him with an unsleeping energy” (609d). To be truly thorough, wouldn’t he have to consider every, or at least many more, examples of vicious dying people to see if their wickedness does indeed dissolve their soul?
One possible response to objection 4: Brown argues that, in the face of a far-reaching demand,
“Plato’s argument adopts a reasonable, heuristic strategy: it considers the most plausible, most widely accepted scenario. If that scenario for the destruction of the soul fails to harmonize with the available evidence, then we have some reason for thinking that no such account of the destruction of the soul should be accepted. When we follow such a procedure, our conclusion is not ironclad, but we have not begged the question or proceeded without reason.” (309)
Plato’s portrait of the tyrant shows us a paradigm case, drawn from real life examples to be sure, of an unjust soul and uses this paradigm case a testing ground: if such a rotten soul is not destroyed but, on the contrary energized, wouldn’t less evil souls avoid dissolution as well?
So those are some objections with possible responses. I certainly haven’t covered all the bases. But hopefully the above has given you enough information to help you critically engage with the argument and avoid straw men. I don’t think the argument deserves to be so quickly dismissed. It certainly has its share of problems. But it is a plausible argument for the immortality of the soul out and can be improved and defended. Indeed, the whole point of reading Plato is to continue the dialogue he presents us. So the above analysis should be taken to that end: continuing the dialogue on this most important of topics, the nature and destiny of the human soul.
In the next post (here) I will present a different approach to the argument, one informed by Plato’s amazing Myth of Er that follows it. This approach takes the immortality discussed in the argument to be something we can achieve in this life if we live a certain way.
 For a full exposition and defense of Plato’s argument, see Brown’s excellent article “A Defense of Plato’s Argument for the Immortality of the Soul at Republic X 608c-611a” in Essays on Plato’s Psychology, edited by Ellen Wagner (Maryland: Lexington, 2001), p. 297. Hereafter, page number.
 Annas, Introduction to Plato’s Republic (Oxford: Oxford University press, 1981), p. 345.
 Plato, in his dialogue The Laws (Book X), has his character The Athenian Stranger say the following which serves as a nice overview of the soul’s range of powers: “Yes, very true; the soul then directs all things in heaven, and earth, and sea by her movements, and these are described by the terms—will, consideration, attention, deliberation, opinion true and false, joy and sorrow, confidence, fear, hatred, love, and other primary motions akin to these; which again receive the secondary motions of corporeal substances, and guide all things to growth and decay, to composition and decomposition, and to the qualities which accompany them, such as heat and cold, heaviness and lightness, hardness and softness, blackness and whiteness, bitterness and sweetness, and all those other qualities which the soul uses, herself a goddess, when truly receiving the divine mind she disciplines all things rightly to their happiness; but when she is the companion of folly, she does the very contrary of all this.” (translated by Jowett). The soul’s capacity to move itself is also discussed in Laws X as well as Phaedrus (245c-246a).
 Read Connolly’s analysis of the Phaedo arguments at the Internet Encyclopedia of Philosophy entry on Plato: Phaedo.
 This formulation is taken, with some minor editing, from K.M. Nielsen’s article “Did Plato Articulate the Achilles Argument?” found in The Achilles of Rational Psychology, eds. Lennon and Stainton (Springer Publishing, 2008), p. 26.
 Plato, Phaedo. Translated by Brann, Kalkavage, and Salen (Newburyport: Focus Publishing, 1998), p. 56.
 Seth Benardete, Socrates’ Second Sailing: On Plato’s Republic(Chicago: University of Chicago Press, 1989), p. 223.
 This overview is taken with minimal editing from Shield’s essay “Simple Souls” in Essays on Plato’s Psychology, edited by Ellen Wagner (Maryland: Lexington, 2001), p. 146. Hereafter, page number.
 Ibid., taken with some minimal editing from p, 147.