Jonathan is overweight and considered morbidly obese. He frequents fast food resteraunts for most meals, and ignores doctors repeated warnings that, with every meal, he decreases his chance of living. He keeps eating fast food because it tastes much better than food he would make himself.
Tamara smokes marijuana every day. Despite the fact that it is illegal, she likes to smoke it because it helps her concentrate and relaxes her. She is not familiar with some of marijuana's health risks but obviously knows the legal risks. She continiues to buy and smoke marijuana despite this knowlege.
So are these activities in these two actors interests? Some say that eating fast food and smoking marijuana are in these two's interests because (obviously) these two engage in these activities. Others say that the activities are not in these two's best interests and that engaging in the activities is acting against their true interests.
To me, this difference of opinion resides in the conflict between what we mean by interests. Does 'interests' mean 'what one is interested in' or does it mean 'what is best for you'? If the former, then a case can be made that whatever we do because we like to do it is in our interest. We can make the argument that no one knows their interest better than others.
If, by 'interest' we mean 'what is best for someone,' then it is quite obvious that others can know our interests better than we. After all, it makes perfect sense to see that a child may not know what is best for her while a mother might (especially when the child wants candy while the mother pushes vegetables).
This question undergirds much of political theory. The question generally is not, or should not be, framed in an all or nothing way. Those who take the extremely libertarian (and really, anarchistic) view that only the individual can judge what is best for her will be foreced into many untenable positions (like the idea that 5 year olds should be able to choose to marry, drive, or do heroin, or that manic depressives should never be protected from suicide against their will). Those who take the opposite position - that others can often know what is best for the individual) have no reason to deny that the government knows what is best for us to read, see, or do.
Rather than being a black or white issue, the question generally is this: how can we recognize that there are occasions where y can know what is in x's best interest while also recognizing y's right to make decisions about her own life without x coming to dominate y? In other words, how can we respect y's ability to lead her own life while recognizing that there may be occasions where x may be in a better position than y (a kid, a mentally deluded person, a drug addict) to judge x's interests.
One can say, and many have, that the difference is in deciphering whether x is "thinking straight." But this hopelessly begs the question because, in general, we judge "thinking straight" by the conclusion come to. If one is against pornography, then there is no way that the urge to consume pornography can be seen as "thinking straight." If one is against suicide, then there is no way that a person who wants to commit suicide (however rational they, or their reasons, might be) will be seen as "thinking straight." Thus, "thinking straight' as a criterion is too nebulous and up to personal opinion to be of value.
So is saying that something is in x's best interest if and only if it is justifiable. Justifiable to whom? And what is and is not justifiable depends on who the proposition is being argued to. If it is the anti-pornography judge, then arguments for pornography can never be justifiable (like they would be to one who sees nothing wrong with pornography). Different people see different things as justifiable, and there is no cosmic "justifiability" principle.
So how can we tell whether x is acting in her interest? I think the issue still comes back to what we mean by her interest. I generally take the side that in the absence of evidence that x has sometype of mental deficiency that grossly inhibits her ability to make an objective (or close to) decision, then we lack any ground to say that x is incapable of ascertaining her own interest. (In this case, children, drug addicts, and those suffering from mood or emotional disorders are all considered mentally deficient in this way. Everyone recognizes this of the latter two groups, but children can be seen as mentally deficient in the sense that they lack the intellectual capacity to make fully informed decisions.)
Further, I take the stance that, if a person is seen as mentally deficient based on psychologicl data, we must further ask about the gravity of the decision x is about to make. Unless her decision is one that will have drastic personal consequences of the type she cannot adequately assess, we should let her do as she pleases. Only when what she is about to do has drastic consequences (engage in consumption of a lethal drug, commit suicide, get into an otherwise dangerous and potentially life changing situaiton), should we interfere and prevent her.
Put differently, the two criteria I think should be used in deciding whether x can accurately appraise her interests and whether to intervene are (a) whether the person can justly be seen as mentally incapable of acting objectively or near-objectively; and (b) whether the "interest" in question has potentially disasterous and life-changing cosnequences.
Following this rule would make it so that we can recognize that the child is not capable of deciding to marrry but the adult can. We could see that an adult can decide for herself whether to smoke marijuana but may not be able to judge whether she should continue injecting heroin or cystal meth. We could achieve a libertarian end of "live and let live" without getting to the point of "live and let die."