Consider the following story:
I like chocolate ice-cream. In fact, I like chocolate ice-cream very, very much. But I know that it is not good for me to eat chocolate ice-cream, particularly when I scoop it onto fudge brownies and dowse the whole affair with chocolate syrup.
One evening I return home from campus and consider fixing just such a chocolate ice-cream sundae. I weigh all the reasons for it (they are delicious and I am hungry) and all the reasons against it (it's really, really fattening, and I am already too fat). I decide that, all things considered, it would be better for me to abstain from having the chocolate ice-cream sundae.
I then calmly, deliberately, and intentionally go to the kitchen, fix, and eat a large chocolate ice-cream sundae.
This is the problem of Akrasia, sometimes also called the problem of Weakness of the Will. Where to have autonomy is to enjoy self-control, akrasia is a curious failure of self-control--curious, that is, because although it might be tempting to say that I was overcome by desire, I wasn't. I intentionally made and ate the sundae; there was nothing frantic or wanton in my action. Yet I acted so after consciously deliberating about what would be best to do and deciding that it would be best to not have the sundae. If asked, “But didn't you just conclude that you shouldn't have the sundae?”, I would respond, between mouthfuls of sundae, “Yes, that is quite correct. But here we are anyway. I'm as surprised about it as you.”
Thus it seems that my actions contradict my reasons for them. Yet I am not raving or mad.
Intentionally performing an action the agent judges worse than an available and incompatible alternative suggests self-deception and perhaps irrationality. If actions are evidence of dispositions and dispositions reliably indicate beliefs, then the agent apparently holds the contradictory belief that P and not P: “I shall refrain from acting yet I shall so act.” This is the problem of weakness of will or akrasia:
What in Anglo-Saxon philosophical circles is called the problem of weakness of will concerns what worried Socrates: the problem of how an agent can choose to take what they believe to be the worse course, overcome by passion. The English expression would not, or at least not primarily, bring this sort of case to mind, but rather such examples as dilatoriness, procrastination, lack of moral courage and failure to push plans through. The Greek word 'akrasia', on the other hand, means 'lack of control', and that certainly suggests the Socratic sort of example. [Gosling (1990), p. 97]
The phrase 'lack of control' should be taken literally. Akrasia is problematic because the weak of will or incontinent somehow fail to act as they themselves think they should--it is as if they are not the authors of their own actions, which makes the intentionality with which they act all the more puzzling.
The tension between best judgment and intentional action the akrates presents, a tension wholly lacking in those who enjoy abundant kratos or power of self-control (enkrateia), is so great Socrates concluded it was simply absurd. Apparent cases are impossible, since “no one who either knows or believes that there is another possible course of action, better than the one he is following, will ever continue on his present course when he might choose the better.” [Protagoras 358c]. Yet Aristotle famously dismisses Socrates' conclusion, since it “contradicts the plain phenomena.” From our translation of Book VII,
We might be puzzled about what sort of correct supposition someone has when he acts incontinently.*
First of all, some say he cannot have knowledge [at the time he acts]. For it would be terrible, Socrates used to think,* for knowledge to be in someone, but mastered by something else, and dragged around like a slave.* For Socrates used to oppose the account [of incontinence] in general, in the belief that there is no incontinence; for no one, in Socrates’ view, supposes while he acts that his action conflicts with what is best; our action conflicts with what is best only because we are ignorant [of the conflict].*
§2 This argument, then, contradicts things that appear manifestly.* If ignorance causes the incontinent person to be affected as he is, we must look for the type of ignorance that it turns out to be; for it is evident, at  any rate, that before he is affected the person who acts incontinently does not think [he should do the action he eventually does].*
§3 Some people concede some of [Socrates’ points], but reject some of them. For they agree that nothing is superior to knowledge, but they deny the claim that no one’s action conflicts with what has seemed better to him. That is why they say that when the incontinent person is overcome by pleasure he has only belief, not knowledge.
§4 If, however, he has belief, not knowledge, and the supposition that [1146a] resists is not strong, but only a weak one, such as people have when they are in doubt, we will pardon failure to abide by these beliefs against strong appetites. In fact, however, we do not pardon vice, or any other blameworthy condition [and incontinence is one of these].
§5 Then is it prudence that resists, since it is the strongest? This is absurd. For on this view the same person will be both prudent and incontinent; but no one would say that the prudent person is the sort to do the worst actions willingly. Besides, we have shown earlier that the prudent person acts [on his knowledge], since he is concerned with the last things, [i.e., particulars] and that he has the other virtues.*
§6 Further,* if the continent person must have strong and base appetites,  the temperate person will not be continent nor the continent person temperate. For the temperate person is not the sort to have either excessive or base appetites; but [the continent person] must have both. For if his appetites are good, the state that prevents him from following them must be base, so that not all continence is excellent. If, however, the appetites are weak and not base, continence is nothing impressive; and if they are base and weak, it is nothing great.
§7 Further, if continence makes someone prone to abide by every belief, it is bad, if, for instance, it makes him abide by a false as well [as true] belief. And if incontinence makes someone prone to abandon every belief, there will be an excellent type of incontinence. Take, for instance,  Neoptolemus in Sophocles’ Philoctetes.* For he is praiseworthy for his failure to abide by [his promise to tell the lies] that Odysseus had persuaded him [to tell]; [he breaks his promise] because he feels pain at lying.
§8 Further, the sophistical argument is a puzzle. For [the sophists] wish to refute an [opponent, by showing] that his views have paradoxical results,* so that they will be clever in encounters.* Hence the inference that results is a puzzle; for thought is tied up, whenever it does not want to stand still, because the conclusion is displeasing, but it cannot advance, because it cannot solve the argument.* §9 A certain argument, then, concludes that foolishness combined with incontinence is virtue. For incontinence makes someone act contrary to what he supposes [is right];  but since he supposes that good things are bad and that it is wrong to do them, he will do the good actions, not the bad.
§10 Further, someone who acts to pursue what is pleasant because this is what he is persuaded and decides to do* seems to be better than someone who acts not because of rational calculation, but because of incontinence. For the first person is the easier to cure, because he might be persuaded to act otherwise; but the incontinent person illustrates the [1146b] proverb ‘If water chokes us, what must we drink to wash it down?’ For if he had been persuaded to do the action he does, he would have stopped when he was persuaded to act otherwise; but in fact, though already persuaded to act otherwise, he still acts [wrongly].
§11 Further, is there incontinence and continence about everything? If so, who is simply incontinent?* For no one has all the types of incontinence, but we say that some people are simply incontinent.
§12 These, then, are the sorts of puzzles that arise.* We must undermine some of these claims, and leave others intact; for the solution* of the puzzle is the discovery [of what we are seeking].
Let us assume Aristotle is correct: Conceptual difficulties notwithstanding, akrasia is a puzzling yet common feature of human agency. Solving the puzzle requires explaining how the akrates' intentional action can deviate so remarkably from her best judgment. How, that is, does it happen that the akrates lacks self-control for actions she herself presumably controls? Is the akrates fundamentally irrational? Self-deceived? Temporarily insane? Given what Aristotle argues in Book VII of Nicomachean Ethics, how does he solve the puzzle of akrasia?