Imagine that in front of you there are two exactly similar objects. Because they are in different places, they are different objects. But is the fact that they are in different places the sole reason for their being different objects?
For centuries philosophers have debated this question. For reasons that are psychological rather than physical, we are inclined to think that the difference between the two objects does not boil down to their being in different places; there has to be another difference. But what could that be?
It has been argued that the two objects, in addition to being in different places, are different substances.
What does this mean? The concept of substance is rooted in the way we think. (I’m not sure of the semantic extension of “we,” but it certainly includes those who are engaged in scientific discourse.) Our grammar reflects our logic, and the primary relation of both grammar and logic is the relation between a subject and a predicate. If we project this relation into the physical world, it becomes the relation between a substance and its properties.
The scholastic philosophers of the middle ages jumped on this possibility and declared substance (or matter) to be
- that which is different in things with the same properties.
At the same time, however, they had to concede that since substance by itself (the subject without predicates) lacks properties, one substance cannot be different from another. And so they also declared substance (or matter) to be
- that which is the same in things with different properties.
To escape this quandary, some philosophers have invented the property of being “this very object.” According to them, two exactly similar objects in different places are different not only because they are in different places but also because one has the property of being “this very thing” while the other has the property of being “that very thing.” In situations like the one envisaged here, however, demonstrative determiners like “this” and “that” distinguish things by pointing at them, and this is the same as distinguishing things by their positions. Instead of solving the problem, these philosophers merely restate it.
Thanks to quantum mechanics, we know now that there is neither a substance that individualizes (is distinct in things with the same properties) nor a property that merely individualizes (like being this very object). When the time is right we will return to this issue.