Rogue VI (Detail) This work "began with a study of the effect of changing the speed of deep water ocean waves traveling through a region with current eddies. It is a numerical ray path simulation of the flow of wave energy as it negotiates a region of current eddies, which refract the waves." Digital art by Eric J. Heller.

In case you decided that, yes, this is indeed the sole reason, did you consider the consequences?

If you think of the two objects by themselves — out of relation to each other as well as any other object — then you are left with no differences at all, for the position of an object is always relative to another object. Positions are spatial relations. (Imagine a universe containing a single object, and then try to tell me where it is. You can't.) But if there are no differences at all, then what there is in front of you is not two objects in two places — this is one "two" too many — but one and the same object in two places. Philosophically speaking, the two objects by themselves are identical not in the weak sense of qualitative identity (exact similarity) but in the strong sense of numerical identity — they are the same thing.

If you want to avoid the conclusion that two exactly similar objects are one and the same object in two places, then you need to point out at least one difference (over and above their different positions).

It has been suggested that there is property (called "thisness") with the amazing capacity to distinguish things without distinguishing them: if in front of you there are two exactly similar objects, one has the property of being this very thing whereas the other has the property of being that very thing. Unfortunately, demonstrative pronouns like "this" and "that" distinguish things by pointing at them, and this is the same as distinguishing them by their positions. Thus nothing is accomplished by postulating a property like "thisness."

If the difference between two exactly similar objects does not reside in their properties, then it has to be a difference between their substances.

What do we mean by "substance" (or "matter," for that matter)?

The scholastic philosophers of the middle ages defined matter (or substance) as

and again as

Funny.

Aristotle did a better job when he defined substance as that which cannot be the predicate in a sentence composed of a subject and a predicate. The grammatical relation between a subject and a predicate reflects a logical relation that is built into the way we think. If we think of objects "out there" as substances with properties, then we project into the world a relation whose origin is in the mind.