Like animals, humans have built-in instincts. Unlike animals, we have the knowledge of good and evil. The human mind is able to determine possibilities among circumstances. In its totally depraved state (apart from God's intervention), man does not possess the ability to choose selflessly even when everything lines up with a choice of that nature.
As well, the moral law of God does not give man this power; it only affords him the conviction of sin, a desire to rebel against it, and the awareness of his inability to obey it. (Romans.7:7-11) This is why God sent Jesus; God in the form of man. He was the fulfillment of the law by His selfless sacrifice to lay down His own life, that He might send the Holy Spirit to dwell within us, giving us the power to choose what is selfless and pleasing to Him.
The things of the Spirit are understood by the Spirit. (1Cor.2:10-16) So, man without the Spirit is unable to choose what is good based on a pure heart that never seeks its own. With the exception of God's common grace upon mankind (whereby our existence is not even possible without), there is always an ulterior motive when a 'good' choice is made by someone void of life in the Holy Spirit.
We are told not to be conformed to this world, but to be transformed by the renewing of our minds. (Romans.12:2)
Again, this is not possible without the Holy Spirit. For the believers, it is the work of sanctification. A process that begins to utilize the things of God through the mind and received by the Spirit which now inhabits us. Thus, faith comes by hearing the word of God. Our minds are the transfer station of the knowledge of good and evil, now processed by the Holy Spirit. Before, the only thing it could choose was self interest, as the Word states, "They are corrupt, their deeds are vile; there is no one who does good." (Psalm 14:1)
With all humanity, our lifelong processes determines our motives, builds our character, and establishes our very soul. It is in this way, believers are able to test the spirit to see if it is of God. (1John 4:1) We can now make truly good choices and can be confident that we know God's will for our lives...not the specifics...rather, the obedient and available man by the power of God through him declares His will for our lives.
"And Jesus said to him, 'Why do you call Me good? No one is good except God alone.'" (Mark 10:18)