1. Suppose there can be an extended simple.
2. Then there can be an extended simple of any shape and size, including an extended simple S that looks just like a teeter totter.
3. It is possible to put enough weigh on a side of S to cause S to tip.
4. If S has no proper parts, then it is not possible put enough weigh on a side of S to cause S to tip (as S would have no sides).
5. Therefore, S has proper parts, which contradicts the stipulation that S is an extended simple.
The weakest link seems to me to be (4). However, it is difficult to see how S could be caused to tip one way rather than another unless the cause acts on *part* of S, rather than on S as a whole.