What is the most plausible complete answer to the problem of personal identity?

While at university I remember reading a bit about the problem of personal identity. It seems clear that a river is the same river from one end of the river to the other end of the river. However, the water molecules in the river change completely along its length. <br><br>Similarly, the human body's cells also change over time, with some studies suggesting that 30% of cells turn over within a 2-4 year timeframe; other studies have found that certain cells in the brain and other organs of the body never change even in a 100 year old person.<br><br>So what exactly makes a human person the same person from birth to death? It is not the cells in the body. Perhaps it is the soul that determines personal identity, but then the soul would need to be defined in a more objective way than the traditional Platonic view of it. Additionally, not all people believe in souls, so what is the alternative?<br><br>I think this is a particularly important issue when it comes to legal matters too, as it would be important to know if a change in personal identity affects legal identity.