The book is Proclus' Elements of Theology.  By 1017 this fifth-century text had likely made its way to Dar al-Hikma.  It would have been slotted in the cabinet beside its more popular (and pseudonymous) abridgment, (Aristotle's) Discourse on the Pure Good, later known as the Book of Causes.[1]

       The Elements of Theology is Proclus' systematic exposition of Neoplatonism.  Its primary deduction — the immortality of the soul.  The Elements may be the most orderly proof of immortality ever composed by a Hellenic philosopher.  In this essay I will present a few Elements which abstract Proclus' argument.  This presentation will pass quickly to a critique.   The critique brings Proclus' image of the soul into focus, as it disentangles the knot of his immortality argument.

       This exercise in criticism is preparation for the modern metaphysical thesis which is to come — and it's an exercise which this author dislikes.   I must raise critical remarks across three ascending chapters before I can pause to bolt the remarks together, as a single thought, in the preparatory conclusion of Chapter 7.   Many readers may imagine the critical material to be unnecessary, or even pointless.  It is neither — but here I think a proper explanation would run too far ahead, so I'll refrain.  Chapter 7 will come soon enough.  Only, here especially I must beg for the reader's patience and trust.

We take up the seamed volume.  Leather opens to paper, paper opens to words.

       The text of Proclus' Elements consists of two hundred and eleven Neoplatonic propositions, organized by a method similar to that which mathematicians use in the construction of mathematical proofs.[2]  When a mathematician wants to prove a novel theorem, he uses in his proof theorems which have been proved before.  Those axioms are foundational, in the sense that they are uncontroversial and accepted as authoritative by the mathematical community.

       In the same spirit Proclus starts at first principles, intuitively.  He builds on this foundation, citing his foundational propositions as the authorities for subsequent arguments.  Proclus may have chosen his method with a mathematical structure in mind.[3]   Be that as it may, the work is ordered logically.[4]  He composes each proposition of two parts:  an argument and a conclusion.  (In mathematical terms, the argument is analogous to a proof, and the conclusion to a theorem proved axiomatically.)

       The conclusion comes first.  Proclus states it in a single sentence.

       After the conclusion comes the argument, in which Proclus deduces the conclusion he has just stated.  Each argument is one paragraph in length.

       A few choice propositions will make clear Proclus' method.  Ten of the 211 propositions are reprinted below.[5]  These particular propositions constitute a significant portion of Proclus' argument for the soul's indestructible and imperishable nature.  The propositions are cogent, although numbered in a way which puts the selected ten slightly out of order when read in isolation.  For this reason I have moved two propositions forward.  So ordered the selected ten state much of Proclus' case.

       Here are the ten propositions, reprinted in full, with notes.  Afterwards, a twenty-first-century critique of Proclus' noble fifth-century thoughts.

Ten propositions from Proclus' Elements of Theology:

Prop. 33.  All that proceeds from any principle and reverts upon it has a cyclic activity.

For if it reverts upon that principle whence it proceeds (Prop. 31),[6] it links its end to its beginning, and the movement is one and continuous, originating from the unmoved and to the unmoved again returning.  Thus all things proceed in a circuit, from their causes to their causes again.  There are greater circuits and lesser, in that some revert upon their immediate priors, others upon the superior causes, even to the beginning of all things.  For out of the beginning all things are, and towards it all revert.[7]

Prop. 17. Everything originally self-moving is capable of reversion upon itself.

For if it moves itself, its motive activity is directed upon itself, and mover and moved exist simultaneously as one thing.  For either it moves with one part of itself and is moved in another; or the whole moves and is moved; or the whole originates motion which occurs in a part, or vice versa.  But if the mover be one part and the moved another, in itself the whole will not be self-moved, since it will be composed of parts which are not self-moved:  it will have the appearance of a self-mover, but will not be such in essence.  And if the whole originates a motion which occurs in a part, or vice versa, there will be a part common to both which is simultaneously and in the same respect mover and moved, and it is this part which is originally self-moved.  And if one and the same thing moves and is moved, it will (as a self-mover) have its activity of motion directed upon itself.  But to direct activity upon anything is to turn towards that thing.  Everything, therefore, which is originally self-moving is capable of reversion upon itself.[8]

Prop. 15. All that is capable of reverting upon itself is incorporeal.

For it is not in the nature of any body to revert upon itself.  That which reverts upon anything is conjoined with that upon which it reverts:  hence it is evident that every part of a body reverted upon itself must be conjoined with every other part — since self-reversion is precisely the case in which the reverted subject and that upon which it has reverted become identical.  But this is impossible for a body, and universally for any divisible substance:  for the whole of a divisible substance cannot be conjoined with the whole of itself, because of the separation of its parts, which occupy different positions in space.  It is not in the nature, then, of any body to revert upon itself so that the whole is reverted upon the whole.  Thus if there is anything which is capable of reverting upon itself, it is incorporeal and without parts.[9]

Prop. 16. All that is capable of reverting upon itself has an existence separable from all body.

For if there were any body whatsoever from which it was inseparable, it could have no activity separable from the body, since it is impossible that if the existence be inseparable from bodies the activity, which proceeds from the existence, should be separable:  if so, the activity would be superior to the existence, in that the latter needed a body while the former was self-sufficient, being dependent not on bodies but on itself.  Anything, therefore, which is inseparable in its existence is to the same or an ever greater degree inseparable in its activity.  But if so, it cannot revert upon itself:  for that which reverts upon itself, being other than body (Prop. 15), has an activity independent of the body and not conducted through it or with its co-operation, since neither the activity itself nor the end to which it is directed requires the body.  Accordingly, that which reverts upon itself must be entirely separable from bodies.[11]

Prop. 43. All that is capable of reversion upon itself is self-constituted.

For if it is by nature reverted upon itself, and is made complete by such reversion, it must derive its existence from itself, since the goal of natural reversion for any term is the source from which its existence proceeds (Prop. 34).[12]  If, then, it is the source of its own well-being, it will certainly be also the source of its own being and responsible for its own existence as a substance.  Thus what is able to revert upon itself is self-constituted.[13]

Prop. 46. All that is self-constituted is imperishable.

For if it be destined to perish, it will then desert itself and be severed from itself.  But this is impossible.  For being one, it is at once cause and effect.  Now whatever perishes is in perishing severed from its cause:  for each thing is held together and conserved so long as it is linked with a principle which contains and conserves it.  But the self-constituted, being its own cause, never deserts its cause since it never deserts itself.  Therefore all that is self-constituted is imperishable.[14]

Prop. 49. All that is self-constituted is perpetual.

For anything which is not perpetual must be so in one of two ways, either as being composite or as existing in another (Prop.48).[15]  But the self-constituted is simple, not composite (Prop. 47),[16] and exists in itself, not in another (Prop.41).[17]  It is therefore perpetual.[18]

Prop. 83. All that is capable of self-knowledge is capable of every form of self-reversion.

For that it is self-reversive in its activity is evident, since it knows itself:  knower and known are here one, and its cognition has itself as object; as the act of a knower this cognition is an activity, and it is self-reversive since in it the subject knows itself.  But if in activity, then also in existence, as has been shown:  for everything whose activity reverts upon itself has also an existence which is self-concentrated and self-contained (Prop. 44).[19]

Prop. 186. Every soul is an incorporeal substance and separable from body.

For if it know itself, and if whatever knows itself reverts upon itself (Prop.83), and what reverts upon itself is neither body (since no body is capable of this activity [Prop.15]) nor inseparable from body (since, again, what is inseparable from body is incapable of reversion upon itself, which would involve separation [Prop.16]), it will follow that soul is neither a corporeal substance nor inseparable from body.  But that it knows itself is apparent:  for if it has knowledge of principles superior to itself, it is capable a fortiori of knowing itself, deriving self-knowledge from its knowledge of the causes prior to it.[20]

Prop. 187. Every soul is indestructible and imperishable.

For all that is capable of being in any way dissolved or destroyed either is corporeal and composite or has its being in a substrate:  the former kind, being made up of a plurality of elements, perishes by dissolution, while the latter, being capable of existence only in something other than itself, vanishes into non-existence when severed from its substrate (Prop. 48).[21]  But the soul is both incorporeal and independent of any substrate, existing in itself and reverting upon itself (Prop. 186).  It is therefore indestructible and imperishable.[22]

Proposition 187 culminates Proclus' case for the soul's indestructible and imperishable nature.  Proclus will proceed to interpret this result as support for a theory in which the soul undergoes an unceasing cycle of reincarnations.[23]

Ten propositions from Proclus' Elements have now been arranged for display, propped on velvet steps as it were.  It's time to subject these propositions to a test of truth — an unfettered critique of Proclus' immortality argument.