The Arminian belief that man is naturally unable to save himself.  The Calvinist view of total depravity actually has some of the same idea behind it.  The difference lies in the role of the sovereignty of God and man's free will.

Under the definition of natural inability, man cannot save himself but God offers prevenient grace which opens the door for man to choose God.  So the term natural ability is slightly misleading as God is still required for man to choose Him.

Scripture makes it clear that man will not seek God on his own because of our sinful nature.  Natural ability does not imply that man chooses to follow Jesus without God's work.  Instead, God opens the door and each person chooses for themselves whether or not to walk through that door.  This uses their "natural ability" or free will after God has already extended grace first.

In the Calvinistic system man cannot and will not choose God but God instead unconditionally elects the believer.