Plato and Aristotle: First Essay
Wylie Breckenridge
In the Phaedrus Plato presents yet another argument for the immortality of
the soul. Many scholars think Plato himself accepted the argument as
sound. Discuss the argument in detail. Evaluate it both from the point of
view of its conception of the soul and its validity/soundness.
The argument that Plato gives is complicated, so I will begin by stating, with detailed
justification, my interpretation of it. I will then evaluate its validity and soundness,
commenting in the process on what the argument suggests about Plato's conception of
the soul. I have used the Walter Hamilton translation of the Phaedrus, published by
Penguin1.
The Argument
I find the argument that Plato presents difficult to follow, largely because of (i) its
structure, and (ii) what it assumes of the reader. On the first point, the main argument
contains several sub-arguments that are not clearly marked out as such nor clearly
linked together. Moreover, they are presented in an inconsistent way - some first stating
premises and then arguing to a conclusion, others first stating a conclusion and then
supporting it with premises and argument. This is not a logical flaw, but it makes for
difficult reading. At one stage he offers, without explanation, a sub-argument whose
relevance only becomes clear once it is viewed as an alternative to a sub-argument
given elsewhere. Again, this is not a logical flaw, but it makes it hard to read. On the
second point, Plato's argument, like any other, aims to persuade a specific audience, and
so it assumes a certain way of speaking and a certain set of background beliefs. It thus
leaves some premises unstated, and uses various terms without elucidation. To me,
however, these premises need to be made explicit and the terms need to be clarified.
I have had, therefore, to interpret Plato's argument, and I present my interpretation
below. My guiding principle has been one of charity: to construct an argument that
remains true to Plato's, and yet has the greatest chance of being convincing to me.
(Rather than being a deliberate decision, adopting such an approach seemed to come
naturally - whenever a section of the argument seemed to be flawed, my inclination was
to think that I had misread or misinterpreted Plato, and I looked for a way of reading it
that removed the flaw.) I have used language that I can understand, I have presented it
in an order that I can easily follow, I have made explicit some of Plato's unstated
premises, and I have left out anything not directly relevant to the progress of the main
argument.
Here is my interpretation (At the back of the essay I have attached a copy that can be
torn off for ready-reference) :
1.
2.
1
All motion is caused by motion.
Therefore, if the motion of an object is not caused by its own motion, then it
must be caused by the motion of something else.
Plato (c.386-380 BC) Phaedrus, trans. Walter Hamilton, Phaedrus and Letters VII and VIII,
Harmondsworth: Penguin, 1973.
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3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
The cause of any motion can always be traced back to a single thing which is
the cause of its own motion. Such a thing is called the 'prime origin' of motion.
If the prime origin of motion were to cease to exist, all motion in the universe
would stop.
For the prime origin of motion to come into existence, its motion must be
caused by the motion of something else. But then it would not be the prime
origin of motion.
Therefore, if the prime origin of motion were to cease to exist, then it could not
come to exist again.
Therefore, if the prime origin of motion were to cease to exist then all motion
in the universe would cease forever.
Therefore, the prime origin of motion cannot cease to exist.
A thing is alive if and only if it has the power of self-motion.
A thing is alive if and only if it has a soul.
Therefore, self-motion is the nature of soul. That is, soul is the prime origin of
motion.
But the prime origin of motion cannot cease to exist.
Therefore, soul is immortal.
By way of justification, I make the following comments (line numbers refer to my
version of the argument):
Lines 1 to 3
Plato's argument relies on certain properties of a "prime origin" of motion, the existence
of which he takes pretty much for granted. But why there might be such a thing needs
spelling out, and that is what I have done in the first three lines. What argument might
Plato have given? He talks of just two kinds of moving thing - "that which owes its
motion to something else" and "what moves itself". The motion of the former is caused
by something else, while the motion of the latter is caused by itself. Plato makes no
reference to a third kind of moving object - one whose motion has no cause. So he
seems to take it that all motion has a cause. Moreover, he says that a moving thing may
be "the cause of motion in another thing", and of a thing that moves itself that "it is the
source and prime origin of movement in all other things that move" (my italics). The
first claim is that motion is a cause of motion. The second is that motion is (ultimately)
the only cause of motion. So Plato seems to take it that not only is all motion caused,
but that all motion is caused by motion. If we take this as a premise, as I think Plato
would allow, then it follows immediately (line 2) that there are just two kinds of moving
objects according to the cause of their motion, and this would explain Plato's reference
to such a division. How would he argue from there to the existence of a "prime origin"
of motion? The obvious way is an infinite regress argument, like this: If the motion of
object A is not caused by its own motion, then it must be caused by the motion of object
B. If the motion of object B is not caused by its own motion, then it must be caused by
the motion of object C. And so on. To avoid an infinite regress, there must eventually be
an object in the causal chain that is the cause of its own motion. But Plato seems to
believe that the causal chain can always be traced back to the same thing. He claims that
what moves itself is "the source and prime origin of movement in all other things that
move", and later he talks of "the prime origin of motion" (my italics in both cases).
Furthermore, he needs there to be just a single prime origin for his argument to work,
because later he claims (my line 4) that if (any) prime origin of motion were to stop
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existing then the motion of the whole universe would cease, and that only follows if
there is only one such prime origin. Accordingly, I have taken the conclusion of the
infinite regress argument to be that there is a single prime origin of motion (line 3).
Lines 4 to 8
The first major step in Plato's argument is to show that the prime origin of motion
cannot cease to exist. He does not explicitly say so, but he seems to offer two
independent arguments for this. I have decided to include only one of them in my
interpretation. This is because (i) I want the argument to reach its conclusion as swiftly
and as clearly as possible, (ii) I think it’s the case, and I think it can easily be shown,
that the argument I have left out adds no more weight to the main argument than the one
I have included, and (iii) I want to keep this essay as short as possible. The first
argument, the one I have left out, is that the prime origin of motion, to be true to its
nature, must always be in motion and what is always in motion must always exist. I'll
leave it to the reader to consider its relative merits. The second argument, the one I have
included, goes by way of the more general claim that no prime origin of motion can
cease to exist (be it the prime origin of motion or something else), but, again for clarity,
and without detracting from the force of the main argument, I have presented it only in
the special case of motion. Line 4 is very close to the claim made by Plato: that if the
prime origin of motion were to be destroyed, then "the whole universe and the whole
creation would collapse and come to a stop". Line 5 is the best sense I can make of the
following passage: "Now a prime origin cannot come into being; all that comes into
being must derive its existence from a prime origin, but the prime origin itself from
nothing; for if a prime origin were derived from anything, it would no longer be a prime
origin". I can think of two different ways in which the prime origin of motion might
come into existence. First, it could be caused to exist by something else, and then once
it exists it could cause its own motion. Or it could be caused to exist by something else
that simultaneously causes its motion. The first way is compatible with it being the
prime origin of motion, because it remains the cause of its own motion. The second way
is not, because the cause of its motion is not itself but whatever it was that caused its
existence. Since Plato is arguing that coming into existence is incompatible with being
the prime origin of motion, I take it that he thinks of the process in the second way, and
that is how I have presented it in line 5. Lines 6 and 7 then follow from lines 4 and 5.
Line 7 says that the consequence of the prime origin of motion ceasing to exist is that
the whole universe would come to a standstill for ever. This seems to be a reductio ad
absurdum for Plato, because he concludes that the prime origin of motion cannot cease
to exist (line 8).
Lines 9 to 13
Having argued that the prime origin of motion, that which moves itself, cannot cease to
exist, Plato's next step is to argue that the prime origin of motion is the soul (and
therefore that the soul is immortal). This is what he says: "Now, since it has been
proved that what moves itself is immortal, a man need feel no hesitation in identifying it
with the essence and definition of soul. For all body which has its source of motion
outside itself is soulless; but a body which moves itself from within is endowed with
soul, since self-motion is of the very nature of soul. If then it is established that what
moves itself from within is endowed with soul, it inevitably follows that soul is
uncreated and immortal". This passage has caused me much anguish. It appears to be an
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argument for soul being (the) thing that moves itself, and yet right in the middle it
makes the claim that "self-motion is of the very nature of soul" - apparently begging the
question. The only interpretation I can think of that removes this flaw is to take Plato to
be talking about living things. If we take him to be saying that the set of self-moving
objects and the set of soul-containing objects are both identical with the set of living
objects, then we can take his statement "self-motion is of the very nature of soul" to just
be a statement of the identification of two sets, not an identification of the defining
property of the two sets, which he can then go on to do in the final step of the argument.
This is the interpretation I have given in lines 9 to 11. The rest of the argument then
follows.
It is difficult to tell from his argument whether Plato thinks of there being just one soul
that we all share in, or of there being a multitude of individual souls. Perhaps Plato was
unclear himself, or perhaps he is deliberately vague on the point. He has reason to be so,
because the distinction presents him with a dilemma. On the one hand, his argument
only works if there is a single prime origin of motion, and if he is to identify this with
soul then he needs soul to be a single thing. But then, even if he allows that the one soul
can manifest itself in many living bodies, his argument only establishes the immortality
of soul as a totality, not the immortality of each individual manifestation. To offer any
solace to a human concerned about the immortality of his own soul, Plato needs to
establish the immortality not of soul as a whole, but of each individual (piece of) soul.
But his argument cannot do this.
So of all the ways I can think of presenting Plato's argument, the one that I have given is
the one I believe is the most convincing while remaining true to the spirit of the
original.
Evaluation of the argument
I was planning to evaluate the argument first in terms of its validity and then in terms of
its soundness, but now I think that approach would not be fruitful. I think that some of
Plato's steps do not, in fact, follow from previous steps in the argument. But I'm not sure
in these cases whether to say that an invalid inference has been made, or that the
inference is valid but relies upon an unstated false premise. It's a distinction that I find
intriguing, but not really to the point when it comes to deciding how good Plato's
argument is. Instead, I will combine the concepts of validity and soundness into one
concept, and consider instead how convincing the argument is. How convincing an
argument is depends on how acceptable are its individual steps, and this is what I will
analyse below.
I do not believe that steps within arguments are ever absolutely acceptable, nor that
arguments are ever absolutely convincing. The degree to which they are depends very
much on to whom the argument is being presented. It is easy to convince a young child
that Santa Claus exists - a simple statement of the conclusion will probably be accepted.
But it would take a much more elaborate argument to convince an adult. There was a
time when humans found the following argument convincing: "The earth looks flat,
therefore the earth is flat". But very few of us would be convinced by that argument
today. I hope that Plato is right about the immortality of my soul, because I'd like to
know how many of the arguments that I find convincing today are sneered at by people
in 2000 years time.
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So in order to consider how convincing Plato's argument is, I have tried to answer these
two questions: (i) How convincing is it to me?, and (ii) How convincing should it have
been to Plato? By the second I mean, given as much as I know about Plato's background
beliefs about such things as the world and the soul, and assuming him to be a rational
man, should he have found his own argument convincing? I have quick answers to both
questions: (i) not very, and (ii) not very. In what follows I will elaborate.
1.
All motion is caused by motion.
Plato should have accepted this. It agrees with the many cases we observe of moving
things causing other things to move, and I believe it fitted naturally into the Greek view
of the world.
Initially, I did not accept it. When a moving billiard ball strikes a stationary billiard ball
and causes it move, it seems like it's the motion of the first that causes the motion of the
second. In one sense that is true - the event would not have occurred unless the first ball
had moved into contact with the second. But modern physics says that the cause is at a
deeper level - in electromagnetic repulsion between the particles composing the balls.
Looked at in this way it seems to be a case of motion caused by something other than
motion. But physics also says that the electromagnetic forces involved are themselves
caused by motion - the spinning of sub-atomic particles. So it seems we are back to
motion being the cause of motion. The natural question to ask next is, What causes the
spinning of the particles? I understand that physicists take it to be a fundamental fact
about the particles - they just spin. So it looks as though we should think, in the case of
the billiard balls at least, that motion is caused by motion. And I suspect the same could
be said of all other cases. Physics today says that motion is ultimately caused by one (or
more) of four forces, and each of these forces is thought to be caused by the motion (of
some kind) of elementary particles, whose own motion is left unexplained. So I am
prepared to accept line 1.
2.
Therefore, if the motion of an object is not caused by its own motion, then it must
be caused by the motion of something else.
I accept this, and so should have Plato - it follows from line 1. Strictly speaking, if an
object does not cause its own motion, then it only follows that its motion is caused by
some other motion, not necessarily by the motion of some other thing. But its hard to
accept that there might be motion without it being the motion of some thing. So I don't
think this is any objection.
3.
The cause of any motion can always be traced back to a single thing which is the
cause of its own motion. Such a thing is called the 'prime origin' of motion.
I do not accept that this follows from lines 1 and 2. It's natural to think that there must
be some stopping point in the causal chain - some thing (or things) whose motion does
not need to be explained - or else we are left with an uneasy feeling that something
'spooky' is happening. Physics today seems to believe it - they trace the causal chain
back to fundamental particles, whose motion is taken to be a brute fact. Instead of
regarding their motion as being inexplicable, however, Plato thinks of it as being selfcaused, which I find harder to accept than that it is simply uncaused. There are at least
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two problems with the conclusion that Plato draws. Firstly, if object A owes its motion
to object B, and object B is not the cause of its own motion, then it does not follow that
there is an object C that is the cause of the motion of object B. The motion of B might
be caused by the motion of A, so that we have a causal loop. This is a possibility, and
one no harder to accept than that there are objects that cause their own motion (which is
also a causal loop, albeit a very tight one). Secondly, even if the motion of object A can
be traced back to the motion of a single self-moved object X, and the motion of object B
can be traced back to the motion of a single self-moved object Y, it does not follow that
X is the same as Y. There may be more than one "prime origin" of motion.
Plato should have been aware of these problems. If he was not, then I can only suggest
that the idea of a prime origin of motion was so embedded in his background beliefs that
he did not consider the matter carefully enough. It would be like us today giving a quick
argument for why the Earth is round - we might not pay too much attention to the
details because we'd expect people to be easily convinced, if they weren't already.
4.
If the prime origin of motion were to cease to exist, all motion in the universe
would stop.
Plato should have accepted this. It is natural to think that the cause of motion must also
be the sustainer of motion. Once I stop pushing a ball, the ball stops rolling; unless I
keep making an effort to run, my legs will stop moving and I'll fall over.
But I do not accept it, because I believe in Newton's first law of motion. It says that
once set in motion an object will continue to move forever, even once the cause of the
motion is removed. The only reason that the motion would stop is if something acted on
the object to stop it (friction in the case of the first example above, and gravity in the
case of the second).
5.
For the prime origin of motion to come into existence, its motion must be caused
by the motion of something else. But then it would not be the prime origin of
motion.
If Plato accepted this, it was because of the way he thought the prime origin of motion
would come into existence: something would cause it to exist, and simultaneously cause
its motion as well. But, he suggests, that would mean it is not a prime origin of motion.
But I don't think this follows - its existence and motion might initially be caused by
something else, but once it exists it might then become the sustaining cause of its own
motion, and thus a respectable prime origin of motion (i.e. an end of the causal chain).
So I don't think Plato should have accepted it.
Moreover, as I discussed earlier, there is another way the existence might come about:
something might cause the prime origin to exist, and the prime origin might then cause
itself to start moving. It is no less plausible that this should happen simultaneously than
that the one thing should cause both the existence and motion of the prime origin
simultaneously. Viewed in this second way, it seems quite possible that the prime origin
of motion could come to exist without losing its claim to being a prime origin.
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I do not accept this step, because I have no reason to believe that the existence would
come about in one of these ways and not another. And Plato should not accept it either,
unless he has further background beliefs that support the position he takes.
6.
Therefore, if the prime origin of motion were to cease to exist, then it could not
come to exist again.
I accept that this follows from line 5, and so should have Plato.
7.
Therefore, if the prime origin of motion were to cease to exist then all motion in
the universe would cease forever.
I accept that this follows from lines 4 and 6, and so should have Plato.
8.
Therefore, the prime origin of motion cannot cease to exist.
I don’t see how anyone could accept this step. Plato seems to think it impossible that all
motion in the universe might cease forever. It seems unlikely to me that it would happen
in the near future, but certainly a possibility that it might happen at some stage. If Plato
accepted it, then he must have had reasons that are not clear to me.
9.
A thing is alive if and only if it has the power of self-motion.
I accept this, and so should have Plato. It seems reasonable, as long as we take selfmotion in an appropriate sense. There are tricky borderline cases between life and nonlife and self-motion and non-self-motion, but there are also clear-cut cases of each.
None of the things like rocks and chairs that we say are not alive have the power of selfmotion. All of the things like dogs and humans that we say are alive have the power of
self motion (excluding tricky cases like people in comas). Indeed, the power of selfmotion is, and was, taken to be a necessary and sufficient condition for life.
10.
A thing is alive if and only if it has a soul.
Plato should have accepted this. To the Greeks the soul was what gave life to the body,
and so it would have been absurd to suggest a living thing (of the appropriate kind) did
not have a soul. Furthermore, giving life to things was the nature of soul, and so it
would have been equally absurd to suggest anything with a soul was not alive.
I do not want to accept it, because I do not believe in the soul. But in that case, I should
reject the whole argument as pointless, because it would be trying to show the
immortality of something that I don't even think exists. So for the purposes of this
exercise, I should pretend that I have a (modern day) belief in the soul. In that case, I
think I'd agree with the Greeks - having a soul is (pretty much by definition) what
makes the difference between life and non-life. I accept line 10.
11.
Therefore, self-motion is the nature of soul. That is, soul is the prime origin of
motion.
Plato's argument seems to be something like this: Whatever has soul has life; whatever
has life has the power of self-motion; so whatever has soul has the power of self-
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motion. Similarly, whatever has the power of self-motion has life; whatever has life has
soul; so whatever has the power of self motion has soul. Therefore, a thing has soul if
and only if it has the power of self motion. So soul itself must have the power of selfmotion. But there is only one thing with the power of self-motion - the prime origin of
motion. So soul is the prime origin of motion.
This seems like a good argument, but it breaks down in the transition from talk about
things that have a soul to talk about soul itself. (The problem can be easily missed
because of Plato's vagueness about whether there are many souls or just one.) Consider
instead this argument: Whatever has a heart has a kidney, and whatever has a kidney has
a heart. Therefore a thing has a heart if and only if it has a kidney. So hearts themselves
must have kidneys. This is obviously wrong, because hearts can belong to other things,
but they cannot belong to themselves. I think it's the same fault in Plato's argument, but
more difficult to detect because soul can be thought of as an individual thing that can
belong to other things, or as a collective term for individual souls that belong to it. If we
try, however, to say that soul belongs to living things in the same way that soul belongs
to soul, then we are logically obliged say that a living thing is nothing but a collection of
individual souls. I don't think anybody wants to say that.
For these reasons, I do not accept that line 11 follows from lines 9 and 10. I find it hard
to believe that Plato wasn't aware of these difficulties. So if he accepted this step, he
must have had regarded it as just an inadequate argument for something he believed on
different grounds. Perhaps it was the view that the nature of soul is to give life, and the
nature of life is to be capable of self-motion, and so the nature of soul is self-motion.
But he would need to argue for this case more explicitly, and I believe that it too faces
problems. Plato should not have accepted line 11.
12.
But the prime origin of motion cannot cease to exist.
This is a restatement of line 8.
13.
Therefore, soul is immortal.
I accept that this follows from lines 11 and 12, and so should have Plato.
Conclusion
There are some steps that I accept, and that Plato should have accepted too, either
because they are justified by our background beliefs about the world, or because they
are just matters of logic. There are some steps that I do not accept, but that Plato should
have accepted, because they are not justified by my background beliefs, but they are
justified by his. And there are steps that I do not accept because they cannot be justified
by my background beliefs, and that Plato should not have accepted unless he had
background beliefs beyond those that I'm prepared to attribute to him. In summary, the
argument is not convincing to me, and could only have been convincing to Plato if he
had beliefs about the world and the soul that are not made explicit in the argument, and
that are not known to me from elsewhere.
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Appendix: Tear-off copy of the argument
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
All motion is caused by motion.
Therefore, if the motion of an object is not caused by its own motion, then it
must be caused by the motion of something else.
The cause of any motion can always be traced back to a single thing which
is the cause of its own motion. Such a thing is called the 'prime origin' of
motion.
If the prime origin of motion were to cease to exist, all motion in the
universe would stop.
For the prime origin of motion to come into existence, its motion must be
caused by the motion of something else. But then it would not be the prime
origin of motion.
Therefore, if the prime origin of motion were to cease to exist, then it could
not come to exist again.
Therefore, if the prime origin of motion were to cease to exist then all
motion in the universe would cease forever.
Therefore, the prime origin of motion cannot cease to exist.
A thing is alive if and only if it has the power of self-motion.
A thing is alive if and only if it has a soul.
Therefore, self-motion is the nature of soul. That is, soul is the prime origin
of motion.
But the prime origin of motion cannot cease to exist.
Therefore, soul is immortal.
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