Augustine and the Calvinistic tradition in general define the will's freedom, or lack thereof, in relation to sin. Why? Because this is how the Bible defines it. Jesus declared "everyone who practices sin is a slave to sin. ... So if the Son sets you free, you will be free indeed." (John 8:34-36) Augustine understood that before the fall, Adam was "able to sin and able not to sin", that he, as representative of the human race, was in a probationary state, not sealed in righteousness (like the glorified saints).
Likewise regarding man's condition after the fall he said we are in the sad condition of being "not able not to sin" So Augustine understood the Bible to be teaching that Adam (pre-fall) was free in regards to sin's bondage but his willful act rendered his post-fall descendants to be in bondage to corruption; to have a will that is no longer free at all (apart from grace) to make God-pleasing redemptive choices. It is worthwhile to remember this in your discussions about free will, because the historical debate about free will refers to man's condition in sin after the fall.
"To will is of nature, but to will aright is of grace." - Augustine
If man has lost his freedom, and is forced to serve sin, and cannot will good, what conclusion can more justly be drawn concerning him, than that he sins and wills evil necessarily?" - Martin Luther BW pg. 149
"I frankly confess that, for myself, even if it could be, I should not want "free-will" to be given me, nor anything to be left in my own hands to enable me to endeavour after salvation; not merely because in face of so many dangers, and adversities and assaults of devils, I could not stand my ground ; but because even were there no dangers. I should still be forced to labour with no guarantee of success.¦ But now that God has taken my salvation out of the control of my own will, and put it under the control of His, and promised to save me, not according to my working or running, but according to His own grace and mercy, I have the comfortable certainty that He is faithful and will not lie to me, and that He is also great and powerful, so that no devils or opposition can break Him or pluck me from Him. Furthermore, I have the comfortable certainty that I please God, not by reason of the merit of my works, but by reason of His merciful favour promised to me; so that, if I work too little, or badly, He does not impute it to me, but with fatherly compassion pardons me and makes me better. This is the glorying of all the saints in their God." - Martin Luther, The Bondage of the Will (Grand Rapids: Revell, 1957), 313-314.
"...we allow that man has choice and that it is self-determined, so that if he does anything evil, it should be imputed to him and to his own voluntary choosing. We do away with coercion and force, because this contradicts the nature of the will and cannot coexist with it. We deny that choice is free, because through man's innate wickedness it is of necessity driven to what is evil and cannot seek anything but evil. And from this it is possible to deduce what a great difference there is between necessity and coercion. For we do not say that man is dragged unwillingly into sinning, but that because his will is corrupt he is held captive under the yoke of sin and therefore of necessity will in an evil way. For where there is bondage, there is necessity. But it makes a great difference whether the bondage is voluntary or coerced. We locate the necessity to sin precisely in corruption of the will, from which follows that it is self-determined.- John Calvin from Bondage and Liberation of the Will, pg. 69-70