Introduction
I have been putting a lot of thought into what defines my personal morality for some time now. I came up with an idea a few years back, and I think it has survived the test of time and personal scrutiny. It has also passed the scrutiny of several friends with whom I have discussed it. For me all morality boils down to one rule with one caveat.
Rule and Caveat
Rule: Don’t do anything to another person that they consider harmful.
Caveat: Certain individuals (children, mentally incapable, etc) can be deemed by society as having that choice handled by others.
I have come to the conclusion that this statement covers everything I believe in. The rule covers all major crimes that have some type of strict codification: murder, rape, theft, assault, kidnapping, etc. It also means that I cannot force my beliefs on someone else. And importantly, it means that if my dying friend needs help to leave this life, I can provide that help and it isn’t murder because he didn’t consider it harmful.
The caveat covers the case where the person in question is deemed to not have adequate judgment to tell what is harmful. Society already makes this distinction, and so do I. A child doesn’t know some things are bad or doesn’t yet have the maturity or experience to make that decision, so we as a society make it for them. Children aren’t allowed to smoke or drink. There are any number of other restrictions on children. The same points can be applied to aging members of society with dementia, and those members of our society with mental health issues. But we do give some choices to children. And the choices that are handled by the children vs. family or society are determined by various criteria. This caveat is not a hard line drawn in the sand. Its a principle to be followed with judgment.
Justification
My justification seems to follow from the principle that we are self-aware beings. We have choice and free will, or at least the illusion of such. If you define being human in terms of being self-away and having free will, then it naturally follows that anything that constrains this is anti-human. This is where I would squeeze in my idea of morality. I have a free will and you have a free will; if we get in each others way then that is the point of conflict and the only possible point of morality. Why follow any other artificial constructs?
Application of Personal Morality
All laws that do not agree with these moral principles are against them. This is fundamental to the first (and only) rule of my morality. For example, a law that limits two adult gay people from having a relationship is immoral. If these two people are consenting, then their relationship isn’t breaking this moral rule. But a law against it is. This law is forcing onto these two people some other person’s belief systems. These two people do not deem their relationship to be harmful to them, but they do deem this unwarranted intrusion harmful, so the law is immoral.
I think my morality boils down to this: freedom. Pure and simple, freedom from everything and anything unless that anything is harmful to someone else. Should I be able to smoke? Yes. Should I be able to smoke in a crowded restaurant where second hand smoke can cause other people harm? No. Should I be able to worship any deity I choose? Yes. Should I be able to make laws based on those religious beliefs that impact others? No.
Social Impact
Of course absolute rules of morality break down in a society where we have to share some resources for the good of all. And if you carefully read my caveat, I am actually stating that I believe society or family needs some say over who gets full control of their own free will. I am quite aware of the contradiction here. This is a kind of recursive definition; I think if you exam many things in life you will find this type of self-referencing structure. To be blunt I am not sure how to boil this down to a simpler explanation. At some point society has to make a determination about others in that society. I believe that determination should be as liberal as possible. That is to say, that society should try to err on the side of free will as opposed to the side of enforcing restriction. But the line has to be drawn somewhere. And there are obvious barriers that should not be crossed.
What sacrifices to my own free will should I be willing to make to gain the benefits of society? What do I contribute so that I have security and a better life in general? Where do we force upon others these same sacrifices? I live in a country (United States) where I am forced to pay taxes for a number of things I will never use myself, but can be argued help society as a whole. A lot of it I would argue doesn’t help at all or actually hurts us. I think this is where personal morality starts to run into social morality as stated above. Should we have a big military to protect us or a small military, or should the defense of our nation simply be up to individuals? Should we have a central health care system or should health care be the responsibility of each citizen? Where do we draw these social lines? I don’t know where my moral principle draws a line on these issues, or if it even can. This is the one place it really fails me. But that is built in to the rule and caveat. Its OK that it fails here.
Conclusion
In my personal life I think this morality has began to really serve me well. I feel I am less judgmental of some people, that I am more open minded. I feel more at peace with my beliefs and the way they shape the way I live my life. I feel less trouble trying to justify myself. And I also feel that if I make a bad decision that I have the flexibility and freedom to change, to say what I did was immoral and change my opinion or my actions. Carying around the luggage of a strict religious doctrine makes this much more difficult. Carrying around a simple, derived from the nature of existence system seems unencumbering yet fair.

I like your thoughts and largely agree with them. The huge issue is, drawing the line between self-determination and societal imposition. There are some cases where the choice is easy. Sometimes its not so easy.
Save the environment, or save the environment? http://tinyurl.com/messhe
A personal recreational activity, or a societal burden? http://tinyurl.com/yakbh9
Not trying to poke holes in your viewpoint as again, I largely share it. But deciding what is morally right when your dealing with more than one person with different, potentially conflicting values, sometimes there isn’t an easy resolution.