Translation: Before the fall described in chapter 3 of Genesis, humans could choose between sinning and not sinning. Since the fall we are unable not to sin, until we are born again through God's grace—and even then, we often still sin. (Our goal is heaven, where we will not be able to sin.) In 1720, when Hume was 9 years old, Scotland's Thomas Boston popularized this concept with his book Human Nature in Its Fourfold State. (The four states: primitive integrity, entire depravity, begun recovery, and "Consummate Happiness or Misery.")
This doesn't mean that in our natural state, devoid of the power not to sin, we are as bad as we possibly could be: The collective predetermination that is culture (often sprinkled with God's common grace) mitigates our individual predetermination. But the main route to free choice is being born again through Christ: Reason by itself, diseased as it is, will not get us there.
That evangelical understanding differs from the Roman Catholic view ably voiced by George, and it may also suggest why Catholics readily embrace the "natural law" tradition. By doing so they, like Jewish natural law believers, gain more secular academic (and Supreme Court) appointments than evangelicals, who are skeptical of human ability, apart from grace, to apprehend the way things ought to be.
This understanding may also lead to a different kind of hope regarding the possibility of one of the Senate 60 doing a Senator Paine. Differences between the Senate and House bills may prove insurmountable, but politicians are adept at finding reasons to compromise. Reasoned argument has had its day with the 60. We need in one a change of heart.