A reductive account of parthood in terms of causal powers

Analytic philosophers like to reduce. But not much work has been done on reduction of parthood. Here’s an attempt, no doubt a failure as most reductive accounts are. But it’s worth trying.

Suppose that necessarily everything has causal powers. Then we might be able to say:

1. x is a part of y if and only if every (token) causal power of x is a causal power of y.

Some consequences:

2. Transitivity

3. Reflexivity

4. If nothing other than x shares a token causal power with x, then x is mereologically simple and does not enter into composition. Plausibly nothing shares a token causal power with God, so it follows that God is mereologically simple and does not enter into composition.

How does this work for hard cases where parthood is controversial?

Suppose I lose a leg and get a shiny green prosthesis. If the prosthesis is a part of me, then the prosthesis’ power of reflecting green light is a power I have. It seems about as hard to figure out whether the power of reflecting green light is a power that I have as it is to figure out whether the prosthesis is a part of me. So here it is of little help.

Suppose I am plugged into a room-size heart-lung machine. Is the machine a part of me? Well, the machine has the power of crushing people by its weight. It seems intuitively right to say that by being plugged into that machine, I have not acquired the power of crushing people. So it seems that it’s not a part of me.

Is a fetus a part of the mother? Here, maybe the story is some help. The fetus eventually acquires certain powers of consciousness. These do not seem to be powers of the mother—she can be conscious while the fetus is awake. So, once consciousness is acquired, the fetus is not a part of the mother. But earlier, the fetus as the power to acquire these instances of consciousness, and the mother does not seem to, so earlier, too, the fetus does not seem to be a part of the mother. Here the story is of some help, maybe.

However, one doesn’t need all of (1) for some of the applications. The “only if” part of in (1) is sufficient for the heart-lung machine and pregnancy cases.