Controversially, the question of the immortality of the soul/mind arises in Spinoza’s writings, and with it the definition of personal identity. At one point he takes up thoughts about a poet who has, in an Altzheimer’s way, has lost contact with his person. In what sense is the poet still himself? To answer this, Spinoza argues for the existent essence of non-existent modes, a position which Deleuze sums as such:
“A mode’s essence is not a logical possibility, nor a mathematical structure, nor a metaphysical entity, but a physical reality, a res physica. Spinoza means that the essence, qua essence, as an existence. A modal essence has an existence distinct from that of the corresponding mode.” Expression in Philosophy (192)
Despite Deleuze’s assurance that this reality is not mathematical, Spinoza does take recourse to mathematical analogy to make clear his meaning, for instance (cited below), the existence of essence of an infinity of equal rectangles within the essence of a circle (Theorem 35, Euclid) which exist even if only one or even none exist modally.
So the essence of a mind is said to exist within the mind of God, eternally, despite its own limited duration. What this does is give the human mind a kind of eternity, an existence outside of the brief flicker of expression, but what this also does is place that eternal existence in relation to all other essences, of all other things, animate and inanimate, which are also produced by God/Nature. The human mind is eternal in essence as all other things are eternal in essence. But further, (as is shown in the note to EIV39 below), identity itself, our preservation of ourselves as ourselves in duration, is also not guaranteed, and is in fact likely an illusion of perspective. Just as his Spanish poet has died to himself, despite the continuity of his body, unable to recognize even his own writings, we too would only be an infinite series of eternal essences – slight modifications of a rectangle within its circle – defined only by our momentary consonance of parts – both ideational and extended. It is not so much that Spinoza has awarded undue eternity to the human mind, but rather has radically (categorically) undermined the basis upon which the human mind privileges itself to be unique among things in this world, given eternal life, but a life fused with all other things, capable as alien to its own “past” as akin to another thing. I list below relevant passages and definitions to this thinking:
EV29- The human mind cannot be absolutely destroyed with the body, but there remains of it something which is eternal.
(Proof) There is necessarily in God a concept or idea which expresses the essence of the human body, which, therefore is necessarily something appertaining to the essence of the human mind. But we have not assigned to the human mind any duration, definable by time, except insofar as it expresses the actual existence of the body, which is explained through duration, and may be defined by time – that is we do not assign to it duration, except while the body endures. Yet there is something, notwithstanding, which is conceived by a certain eternal necessity through the very essence of God; this something, which appertains to the essence of the mind, will necessarily be eternal.
EIV39note – …But here it should be noted that I understand the Body to die when its parts are so disposed that they acquire a different ratio of motion and rest to one another. For I dare not deny that – even though the circulation of the blood is maintained, as well as the other [signs] on account of which the Body is thought to be alive – the human Body can nevertheless be changed into another nature completely different from its own. For no reason forces me to think that the Body does not die unless it is changed into a corpse. And, indeed, experience seems to urge the opposite conclusion. Sometimes a man undergoes such changes that I should hardly believe that he was the same man. For example, I have heard tell of a Spanish poet who was struck by an illness; though he recovered, he remained so oblivious to his past life that he did not believe the tales and tragedies he had written were his own. He might have been taken for a grown-up infant had he also forgotten his native tongue.
EIp8 – By eternity, I mean existence itself, insofar as it is conceived necessarily to follow solely from the definition of that which is eternal. (Explanation) – Existence of this kind is conceived as an eternal truth, like the essence of a thing, and, therefore, cannot be explained by means of continuance or time, though continuance may be conceived without a beginning or end.
EIp24- The essence of things produced by God does not involve existence. (Corollary)… God must be the sole cause, inasmuch as to him alone existence appertain.
EIp25 – God is the efficient cause not only of the existence of things, but also of their essence.
EIIp8 – The ideas of particular things, or of modes, that do not exist, must be comprehended in the infinite idea of God, in the same way as the formal essences of particular things or modes are contained in the attributes of God. Note – If anyone desires an example to throw more light on this question, I shall, I fear, not be able to give him any, which adequately explains the thing of which I here speak, inasmuch as it is unique; however, I will endeavour to illustrate it as far as possible. The nature of a circle is such that if any number of straight lines intersect within it, the rectangles formed by their segments will be equal to one another; thus, infinite equal rectangles are contained in a circle. Yet none of these rectangles can be said to exist, except in so far as the circle exists; nor can the idea of any of these rectangles be said to exist, except in so far as they are comprehended in the idea of the circle. Let us grant that, from this infinite number of rectangles, two only exist. The ideas of these two not only exist, in so far as they are contained in the idea of the circle, but also as they involve the existence of those rectangles; wherefore they are distinguished from the remaining ideas of the remaining rectangles.