and it is jointly owned by the two neighbours. Can either of them compel the other to repair it?

The answer is: yes, he may compel him, because it is jointly owned and has now fallen into disrepair, so it must be repaired. But if one of them asks the other to widen the river, may he compel him to do so?

The answer is: no, he may not compel him, because this is an enhancement, not the repair of something defective. Thus, if one of their properties expands and he needs more water, and he says, “I want to widen the water inlet so that the flow will increase,” while the other says, “No,” may he compel him?

The answer is: no, he may not compel him, because the other may say, “I have no need for additional water, and you cannot burden me with the expense.” But if the neighbour says, “I will bear the expense,” and the other refuses, may he be compelled?

The answer requires further detail. If he says, “I fear that, if the water inlet is enlarged, it will flood me, and I cannot bear that. The river rises and falls, so I fear that it will flood me. When its inlet is narrow, I can ward it off; therefore, I do not agree,” while the other says, “My land has expanded, and I need more water,” then the matter must be examined in reality. We say: if the possibility raised by the co-owner—namely, the fear that increased water may flood the crops—is a genuine one, then he has the right to refuse. He may say to his companion, “Open a separate river for yourself.” But if this possibility is not genuine, and the river runs at a constant level such that no danger is feared from it, then we compel him, provided that the co-owner undertakes the expense.

His statement, “and the waterwheel,” refers to the well-known waterwheel. If this jointly owned waterwheel falls into disrepair, and one of the two co-owners asks the other to rebuild it, the latter is obliged to do so.