In a previous post, I gave an argument for thinking substance dualism is true as opposed to hylomorphic dualism. To re-cap, the argument goes as follows. I will number the premises slightly differently to avoid confusion later:
(1*) Some disembodied conscious subject psychologically continuous with me continues to exist after death.
(2*) I am this subject.
(3*) The animal associated with me does not continue to exist after death.
(4*) So I am not identical to the animal associated with me.
I explain and defend the argument more fully in the previous post, and explain why I think substance dualism follows.
In conversation elsewhere, I discussed this argument with Prof. Robert Koons and some other friends (Koons' new blog, which I recommend, can be found here). Most of them suggested that we should deny the third premise (3*), and say that the animal actually continues to exist in an immaterial state after death. Prima facie this reply seems obviously incorrect to me. Denying (3*) seems to me to be an instance of going to unreasonable lengths in order to deny a conclusion. I sincerely do not mean that to be rude or arrogant, I just really don't think there is good reason for denying it. Still, I want to repeat some of the comments I made in reply to this proposal. I have two worries about this proposal: First, that it stretches the concept of an animal to the point of absurdity, and second that it is impossible given some basic Aristotelian metaphysics.
The first worry is that this seems to be stretching the concept of an animal quite a bit. I feel I begin to lose a grip on what the essence of an animal is if not something which is material, which has biological organs, biologically functions in a certain way, interacts with a certain environment, is alive, etc. So if hylomorphic dualism entails the possibility of immaterial animals and animals with none of these features, this may actually count as an argument against the plausibility of hylomorphic dualism.
Moreover, suppose at some time t all forms of life in the universe cease to exist but our souls subsist. Then, at least given one scholastic criterion of God's omnipotence i.e. that God has the power to bring about whatever involves no explicit contradiction, God could bring about that there are animals who are never any of the things I listed (material, having biological organs, etc.); for he could create ex nihilo a different world just like this one except the first moment of this world is t. So it would seem hylomorphic dualism not only entails the possibility of immaterial animals, but it also entails the possibility of animals who are never at any time material, living, having biological organs, etc.
This seems to me to count against hylomorphic dualism; at the very least animals must have these material features at some time (most likely when they began to exist). The sense of "must" here is not only statistical, as in "they usually begin in a material state", but rather metaphysical in the sense of, "necessarily (in all metaphysically possible worlds), they begin to exist in a material state." Maybe one could reply that the situation I describe is actually impossible, because somehow it involves a contradiction to suppose an animal is created in an immaterial state; but I do not see any prima facie contradiction in this situation, at least if one thinks it is possible for them to exist immaterially. So until this contradiction is made explicit I do not have a good reason to think there is one.
The second objection I have is based on Aristotelian metaphysics. Traditionally in the Aristotelian "genus-et-differentiam" tree animals will fall under the broad category of material substances, and since genus and specific difference give you the essence of something (in our case, rational + animal), and since animal falls under 'material substance', being material is part of the essence of all animals. But then I take it if you have a certain feature as part of your essence, then necessarily if you exist then you have that feature (thought I don't believe the converse always holds). Hence, it is not possible to be immaterial and still be an animal. This argument can be formalized as follows:
1. The essence of 'man' is 'rational animal'.
2. 'Animal' falls under the genus 'material substance'.
3. If one of the components of the essence of x falls under a genus, then that genus is part of the essence of x.
4. So, 'material substance' is part of the essence of 'man'.
5. But if P is part of the essence of x, then necessarily if x exists then x is P.
6. So, necessarily if man exists then man is material.
That can be made more precise but it should suffice. Let me explain premise 3 since it's the only one that I think might be a bit unclear. To take an example, you might say that 'chair' has as its essence 'three-or-four-legged furniture piece' or something like that. In this case then 'furniture piece' is a component of the essence of the chair. 'Furniture piece' falls under the genus 'substance'; so then premise 3 would in this case imply that 'substance' is part of the essence of the chair. Simple enough.
Now, premise 1 I take to be part of hylomorphic dualism. Premise 2 is the traditional Aristotelian picture (see e.g. Aristotle's De Anima Bk. II 412a2-b6 plus St. Thomas's commentary, as well as Oderberg's 'Real Essentialism' pg. 99). Besides, it would be very weird for an Aristotelian to deny it, since then you will have some some animals falling under 'material substance', others falling under 'immaterial substance', and hence the term 'animal' will be at best an analogical term between us and lower animals. Which seems clearly to not be what is going on anywhere in Aristotle. The rest of the premises I take to either be self-evident or to clearly be true in the Aristotelian framework. But I was quite surprised to find some people denying premise 5, so I will defend it.
Someone might object to premise 5 because they think there are counter-examples. For instance, you might say that the feature of being bipedal is part of the essence of humanity, and yet clearly people who are missing their legs can exist and yet be human. However, I do not think it is part of the essence of humans to be bipedal; for one, if anything, it "flows" from the essence but is not part of it. Essence is to be identified properly speaking with the definition by genus and specific difference (or at least the non-linguistic correlate of this definition), and the genuses above are said to be "virtually contained" in the essence (this is the term Oderberg uses anyway; I prefer to just say they're part of the essence). And being bipedal doesn't show up anywhere in the essence (whereas given the definition "rational animal", "material substance" does show up in the essence). So being bipedal is not part of the essence of humans.
But what about a feature which is presumably essential, like that of being rational. Rationality is part of our essence, yet clearly there are many people who are both irrational and in fact cannot even immediately exercise their rationality. The problem with this proposed counter-example is that it is a misunderstanding of what is meant by "rationality." By rationality is meant having a tendency to exercise our intellect. So, if we analyze the term "rationality", what is essential to being human is having a tendency to exercise our rational capacities, not actually exercising our rational capacities (where tendency is taken in a teleological sense rather than statistical sense). I think Aristotle even says some things like this in the De Anima or somewhere, though to be fair I don't have my copy at the moment so I can't confirm this.
These are some objections against premise 5; but what can be said in its favor? To answer this we need to get clear on what is meant by "essence." On the traditional understanding, or at least any understanding relevant to this discussion, I would contend that premise 5 is definitely true.
When I talk about essence, by "the essence of x" I mean what it is to be x. This is the understanding of Aristotle (the Greek phrase translated as 'essence' in Aristotle literally means 'what it was to be'; see e.g. here where Greek can be loaded on the right). On this definition, it seems to me to clearly follow that for x to exist it must have its essential properties. For in order to be x you must have the essence of x; but in order for x to exist it must be x; so in order for x to exist it must have the essence of x.
Now, maybe one will reply that you can have the essence of x, yet only have the essence of x partially, and thus premise 5 will not be true since you can be missing part of the essence. The problem with this reply is that if you only have the essence of x partially, and the essence of x is what it is to be x, then you can only partially be x, i.e. you can only be x in some non-literal, lesser sense. So if something only possesses the essence of x partially, it is not x (but only partially x). For x to exist (rather than partial-x, say) it must fully be itself (there's no "half-identity" or something), and so to be x one must have the essence of x fully, and thus have all the parts of x's essence. So on the traditional reading of essence coming from Aristotle, 5 is still true.
Maybe though one could give a modified definition of essence, and say something like the essence of x is that which grounds what it is to be x. However, even on this somewhat vague definition, I think it follows that if it is part of your essence to be P then you must be P if you exist. For if the essence of x grounds what it is to be x, then what it is to be x depends on the essence of x (since if A grounds B then B depends on A). Hence, if the essence of x includes being P, then x's being what it is to be x depends on x's being P. But if x's being what it is depends on x's being P, then x's being what it is entails x's being P; i.e. x must be P if x exists. So If the essence of x grounds what it is to be x, then it follows x must be P if x exists. That's a long and somewhat wordy argument, but hopefully it is clear enough. I am pretty confident it could be made more precise and detailed.
All of this aside I think premise 5 is self-evidently true and doesn't really need to be defended, especially given an Aristotelian framework. But even countenancing all of these objections it still seems 5 must be true.
For what it's worth, I don't think either Aristotle or Aquinas would deny any of what I've said here; for instance, I am pretty sure I have read Aquinas agreeing that it is no longer the person who exists after death but only their soul (in one of his commentaries on the Scriptures; maybe someone can find the reference). I only bring this up not as a matter of appeal to authority, but to give evidence that in the Aristotelian framework it is completely natural to agree with my premise at the beginning which says the animal ceases to exist at death. And thus, if one holds to basic Aristotelian metaphysics, one should agree with my conclusion, that we are not animals.