we are morally complete and virtuous individuals if we do as we wish so long as our actions do not harm others and we "kick in" to help the needy when this does not set us back much
This is a reasonably fair way to present the dominant liberal view on the matter. I don't think though that it's a position that really works as it's supposed to.
First, the position is more arbitrary than those holding it imagine it to be. It arose out of a rejection of a more traditional view of morality, in which there exists, outside of our own minds and choices, a real category of "goodness" which a moral person will endeavour to follow.
There is a significant current in Western thought which rejected this traditional view (often traced back as far as the nominalists of the Middle Ages). It seems that what often replaced it is the assertion that value is created instead as an act of our own will.
Value, in other words, doesn't exist independently of me, with my task being to align myself with it. Instead, it is in a kind of "process" of individual will that value is made.
Therefore, the highest moral good will be thought to be a freedom for this "process" of willing.
The significant point to be made here is that the modernist view rests on two assumptions. First, that an entity of "goodness" does not really exist external to us and, second, that value really is established "internally" through some kind of process of asserting our will.
It's difficult to establish the truth of these assumptions in terms of modern science, and it's in this sense that the modernist view is established more arbitrarily than is usually admitted.
Nor does the liberal statement of morality really have the simplicity it suggests. In the physical sciences, knowledge can be reduced to simple formulas like e=mc2. It's possible that modernists want to try to mimic the hard sciences by providing a similarly clear and elegant formula for a science of morality.
The formula provided above, though, is only deceptively simple. It tells us we may do as we wish as long as we don't harm others. How, though, is it to be decided if our actions harm others or not?
Once you open up this question all elegance is lost. For instance, does the formula allow a man to divorce his wife? If he does choose to divorce, it might be objected that his action will harm either his wife or children. Alternatively, if he stays in an unhappy marriage this too might produce harm to others. If the man has a mistress, should any potential harm to her be considered? How can the degree of harm be objectively measured and weighed, especially when there are so many unpredictable outcomes, including the chances of the wife remarrying, the degree of resilience of each of the children and so on.
Which brings me to a final consideration. The liberal formula is less impressive in its practical application than the traditional view. Not only is the harm principle difficult to decide in practice, worse still it isn't usually taken very seriously. In other words, the "do what you want" is given a lot more weight than the "don't harm others".
When I asked, for instance, whether the formula allows a man to divorce his wife, I doubt if many political moderns would really ever consider that the answer might in some cases be no. It is simply assumed that the answer is yes.
Nor does the liberal formula really correspond to our actual experience of virtue. Does doing what we want really make us feel virtuous?
For most people, I expect, virtue is felt when we actually restrain from following a passing want in order to hold to what we feel to be right, or out of a sense of moral integrity.
If, for instance, a married man meets a desirable, flirtatious woman on a business trip, he might well experience a "want" to sleep with her, and he might well reason that as no-one will find out no-one will be hurt.
According to the liberal formula, he should feel a sense of virtue in following his non-harming wants and bedding the woman. In reality, he is more likely to experience a sense of virtue at the moment he chooses to follow a sense of right, rather than a want.