Thanks, but the idea of this line as a recommendation doesn't work with the internal/external distinction I was trying to explain earlier, because the external factors are those that we don't have any control over.
The internal/external thing wasn't supposed to mean nature/nurture. As you noted, both nature and nurture would work as internal factors, determining our will by affecting our desires and beliefs.
Here's an example to show what I was going for: A man is moved by his will to go fishing in a river, but the river has dried up! Here his "will" has been thwarted by his external environment and he is no longer "moved by will alone". He is also limited by the nature of the world around him.
But when it comes to our moral choices and development these external factors are irrelevant, because regardless of our situation it is always possible to what is right in that situation.
So with respect to our moral progression we are "moved by will alone" - our lack of progress is our responsibility and we cannot shrug it off by blaming it on a hostile universe.

(Doesn't mean you're wrong, just that it doesn't fit with the distinction I was trying to explain)