In seeking to explain the difference between moral and natural ability (a concept taught by Jonathan Edwards), taking something we can understand in the physical realm (the realm observable to our senses) and applying the principle to something we cannot see – the hidden desires of the heart. In commenting on Jesus’ words in John 6:44, “No one can come to Me unless that Father who sent me draws him, and I will raise him up at the last day” he writes:

“… Edwards distinguished between moral and natural inability. Because man’s inability is moral and not natural, according to Edwards, the individual is responsible for the choices he or she makes. Here is a simple illustration: in the natural world there are animals that eat nothing but meat. They are called carnivores, from caro, carnis, which means “meat.” There are other animals that eat nothing but grass or plants. They are called herbivores, from herba, which means vegetation. Imagine taking a lion, who is a carnivore, and placing a bundle of hay or a trough of oats before him. He will not eat the hay or oats. Why not? It is not because he is physically or naturally unable to eat them. Physically, he could munch on the oats and swallow them. But he does not and will not, because it is not in his nature to eat this kind of food. Moreover, if we were to ask why he will not eat the herbivore’s meal, and if the lion could answer, he would say, “I can’t eat this food, because I hate it. I will only eat meat.”

Now think of the verse that says, “Taste and see that the LORD is good” (Ps. 34:8), or of Jesus saying, “I am the living bread that came down from heaven. If anyone eat of this bread, he will live forever” (John 6:51). Why won’t a sinful person “taste and see that the Lord is good” or feed upon Jesus as “the living bread”? To use the lion’s words, it is because he “hates” such food. The sinner will not come to Christ because he does not want to. Deep in his heart he hates Christ and what he stands for. It is not because he cannot come naturally or physically.

Someone opposed to this teaching might say, “But surely the Bible says that anyone who will come to Christ may come to him. Didn’t Jesus invite us to come? Didn’t he say, ‘Whoever comes to me I will never drive away’ (John 6:37)? The answer is, “Yes, that is exactly what Jesus said, but it is beside the point.” Certainly, anyone who wants to come to Christ may come to him. That is why Jonathan Edwards insisted that the will is not bound. However, this liberty is what makes our refusal to seek God so unreasonable and increases our guilt. Who is it who wills to come? The answer is, No one, except those in whom the Holy Spirit has already performed the entirely irresistible work of the new birth, so that, as a result of this miracle, the spiritually blind eyes of the natural man are opened to see God’s truth, and the depraved mind of the sinner, which in itself has no spiritual understanding, is renewed to embrace the Lord Jesus Christ as Savior.”

– James Montgomery Boice, Philip Graham Ryken, The Doctrines of Grace, p. 85, 86