Let’s start with an analogy: a river flowing across the countryside. Where the slope is nearly flat, the river meanders, wandering here and there according to the influence of various local factors. Where the slope is more pronounced, the river follows a definite course. With a bit of effort, a primitive farmer can use the river for irrigation. Lacking any kind of pump, though, he’s going to find that not all attempts to harness the river will be successful, and that the most successful approaches all have one factor in common: remembering that water flows downhill.
Morality is like the river, in that there are some circumstances where it is fairly easy to make it become what we want it to be, as well as other circumstances where, do what we will, the “water” is going to follow its natural downhill flow. But if morality is like the river, then what is the landscape that shapes its natural course, and what force of “gravity” pulls it downhill? That one is a little more complicated to explain.