I think the following claim (or some version of it) is used by several political theorists (e.g., Locke and Hegel) to justify property rights:
Investment Principle (IP): If A invests its will in some (previously unowned) thing x, A thereby has an ownership claim to x.
For example, if I carve a statue out of a previously unowned rock, I thereby have an ownership claim to that rock/statue. This seems false to me. I’ve recently argued against this principle (with the argument that follows below), but I’m not entirely satisfied with the argument and I can’t put my thumb on the source of my dissatisfaction. First, the argument:
On a natural reading, IP says that A’s investment of will in some thing x is sufficient to ground A’s ownership claim to x. But notice that IP is also read as saying that A’s ownership claim to x is necessary for A’s investment of will in x. When read this way, the claim is plainly false. Note that an ownership claim to x includes a right to exclude others from the use of x. Note further that the right to exclude others from x does not add anything to the investment of A’s will in x. For example, consider a child building a sand castle on a public beach. In building the castle, the child invests her will in the sand. But granting the child the right to exclude others from that particular section of the beach doesn’t add to the investment of her will in the sand. Of course, it’s true that the child must be able to prevent others from interfering with her actions if she is to complete the investment of her will. But the child’s investment of will need not presuppose her right to exclude others from the beach; it must simply presuppose that others do not have a right to exclude the child from acting upon the sand on the beach. So no ownership claim is necessary for the child to complete the expression of her will. It follows, then, that the investment of A’s will in x is not sufficient to generate a property right in x. IP is false.
In one light, this argument seems right to me and it seems to defeat IP. But in another there seems to be something suspicious is going on here and I’m not sure I’ve addressed IP the right way. I’m not sure why I get this impression. Perhaps its this: consider the statement `If it’s raining (and the ground is uncovered), then the ground is wet’. The obvious way to read this is that the rain is sufficient for the ground’s being wet. By analogy with what I’ve said above, we can also read it as saying that the (uncovered) ground’s being wet is necessary for it to be raining. But intuitively this seems (to me) to be an odd thing to say. Maybe the odd ring derives from thinking about the statement causally (and, in talking about causation, I’m treading on unfamiliar territory); it sounds right to say that the rain is causally sufficient for the ground’s being wet, but it seems wrong to say that the ground’s being wet is causally necessary for the rain.
Similarly, if we read IP as a causal statement—i.e., A’s investment of will in x is sufficient to cause A to have an ownership claim to x—then it sounds odd (indeed, patently false) to read IP as saying that A’s ownership claim to x is causally necessary for A’s investment of will in x. Thus, showing that A’s ownership claim to x isn’t causally necessary for A’s investment in x (as the argument above purports to show) doesn’t yet show that A’s investment in x isn’t causally sufficient to generate A’s ownership claim to x. And it’s not immediately clear why IP shouldn’t be read as a causal claim. (It’s not immediately clear why it should be so read either.) So perhaps the above argument doesn’t work after all. (Although I think it does work if IP should not be read as a causal claim.)
So why might we think that IP is (or isn’t) a causal claim? One reason to think IP isn’t a causal claim is because it seems to be straightforwardly about justification (not causation) of property rights—i.e., A’s ownership claim to x is justified by the fact that A invests its will in x. Indeed, I’m not even sure what it would mean to talk about how property rights are caused. However, it does seem to make sense about my having specific property rights to x being caused; and maybe this is what IP is about. I don’t think so, but it might. In any case, `Because I invested my will in x’ seems to be an equally intelligble answer to two different questions:
1. How did you come to have an ownership claim to x? (Causal question)
2. What justifies your ownership claim to x? (Justification question)
I’m not sure what else to say at this point, so I’ll leave it at that for now.