Thursday, January 31, 2019

Can our cells be substances?

A standard Aristotelian principle says:

  1. No substance is a part of another substance.

I was just struck by how (1) says less than it seems to. One interesting philosophy of biology question is whether our symbiont bacteria are part of us. But:

  1. All bacteria are substances.

  2. We are substances.

  3. We are not bacteria.

  4. So no bacteria are parts of us. (By 1-5)

This argument is fine as far as it goes. But there is a metaphysical possibility that its conclusion leaves open which it is easy to forget.

Let’s grant that our symbiont bacteria are not a part of us. But perhaps their matter is a part of us. In other words, maybe the bacteria are matter-form composites just as we are, but their matter is a part of our matter, whereas their form is not a part of us at all, and hence they as wholes are not parts of us. They merely overlap us in matter.

And the point can be generalized. Before I noticed this point today, I used to think that the Aristotelian commitment to (1) requires us to deny that our cells are substances. But (1) leaves open the possibility that our cells are substances whose matter is a part of us, while the cells as wholes are not parts of us.

I don’t really want to say this. I would like to supplement (1) with this principle which has generally been a large part of my reason for affirming (1):

  1. The matter of one substance is never a part of another substance.

My reason for accepting (6) has been that the identify of the matter is grounded in its substance, and if the matter had its identity doubly grounded, it wouldn’t be one thing, but two, and so it wouldn’t be the same matter in each substance.

In fact, (6) is a special case of a stronger claim:

  1. No two substances have any matter in common.

Here is an argument that establishes (7) directly. Start with this plausible thesis:

  1. No two material substances have all of their matter in common.

But now if (7) is false, then it should be possible to have two plants that have some matter in common. We could further imagine that the non-common matter perishes, but both plants survive. If so, then we would have a violation of (8). So, it’s plausible that if (7) is false, so is (8).

Here is a different line of thought in favor of (7):

  1. Matter is grounded in the accidents of a substance.

  2. Two substances cannot have any accident in common.

  3. If x is entity grounded in a and y is an entity grounded in b and a ≠ b, then x ≠ y.

  4. So, two substances cannot have any matter in common.

So, all in all, while (1) leaves open the possibility of our cells and bacteria being substances and yet having their matter be a part of us, we have good reason to deny this possibility on other grounds.

It would be very neat if one could derive (1) from (7). From (7) we do directly get:

  1. No substance with matter is a part of another substance.

But it would take more argument to drop the “with matter” qualifier.


Helen Watt said...

Doesn't any theory though have to fit conjoined twins (let's say, two heads, one body) who are definitely two self-directed bodily organisms/substances but who also definitely share some matter which is part of both and under the control of both? If some terrible injury left only the shared parts behind and life still seems to be present I agree there seems a problem with saying that both twins are still there - as opposed perhaps to saying that one twin as the centre of control (which, you may not know) has completely 'taken over'. Plus, even non-conjoined twins can share the fetal placenta etc.

Alexander R Pruss said...

That's a good point. But perhaps we can still say that there is a fact of the matter as to which form has which bit of matter under its sway, albeit a fact not knowable by us?

Helen Watt said...

That could be, but it doesn't look like it - Abby and Brittany Hensel, for example, each manage one side of their conjoined body and the sense of touch is also restricted, however seemingly there is a small amount of overlap at the midline. I suppose theoretically they could be controlling very close parts but it does look as if both are controlling both (and then there are parts of the body that aren't sentient or under conscious control- seemingly common organs, shared blood circulating etc).

Alexander R Pruss said...

There are two kinds of control that our form exerts. One kind is purely causal. This kind of control we have both over our bodies and over things in our environment. When I move my arm, my sleeves move as well, even though my sleeves are not a part of me. This kind of control is empirically observable. The other kind is where the form is directly responsible for the characteristic physical behavior of the parts. The reason the carbon nuclei in my body attract the electrons that orbit them is because my form gives them their characteristic behavior. This kind of control is not empirically observable. When a bit of me leaves my body, carbon nuclei continue to attract electrons, as far as we know in exactly the same way. But on an Aristotelian metaphysics, there is a substantial change there, and the carbon nuclei and electrons are under the sway of a different form (or forms). It is the second the kind of control that we would need to look for to see whether a bit of matter is a part of the substance. But since this second kind of control is not empirically observable, as far as we know, we can't figure this out empirically.

Helen Watt said...

What about internal perceptions though? I see what you mean re pushing a part around that need not belong to you (any more than your sleeve) but if Abby and Brittany both simultaneously feel a pinprick, doesn't that look like an area of shared control?

Alexander R Pruss said...

It looks like it, but I don't think it needs to be. You could get an electrode attached to a nerve, and feel a pinprick when electricity is sent along the electrode, but the electrode need not be a part of you.