To assume that because man has ability to love he therefore has the ability to love God, is about as wise as to assume that since water has the ability to flow, it therefore has the ability to flow uphill; or to reason that because a man has power to cast himself from the top of a precipice to the bottom, he therefore has equal power to transport himself from the bottom to the top.
Predestination, Loraine Boettner, p62.
I like when theologians use physics to explain a theologocal concept. Here, Boettner is trying to explain how man has "wholly lost all ability of will to any good accompanying salvation" (WCF 9.3). The effect of sin is likened to a gravitational pull.