Nature tends to one thing only
Just as in the process of reasoning, the principle is that which is naturally known, so in the process of the will, the principle needs to be that which is naturally desired. This must needs be one: since nature tends to one thing only.
The second reason is because, just as in the process of reasoning, the principle is that which is naturally known, so in the process of the rational appetite, i.e. the will, the principle needs to be that which is naturally desired. Now this must needs be one: since nature tends to one thing only. But the principle in the process of the rational appetite is the last end. Therefore that to which the will tends, as to its last end, is one.