11 An ancient conundrum

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.

Go figure.

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.