This is a blog-article I posted today on my blog: http://christianrr.blogspot.com there you can also find the sketch I refer to.

On a blog that I recently visited I was asked where I, as an atheist, take my morals from. This questions also contains in my opinion the question why one should be moral if (s)he does not have to fear Judgment Day.
The concrete question was, why I would not eat my grandmother. The answer to this question lies within self-interest. My wealth and my well-being depends on the fact that I live in a society that is based upon common rules that make a civil interaction between people possible. The German philosopher Immanuel Kant has put the rule of morality very good in his categorical imperative:

"Act only according to that maxim whereby you can at the same time will that it should become universal law."

So, why would an atheist live according to that rule. The reason is quite simple: It is the only logical way to have a moral society. Just if everyone in a society acts according to principles that he or she could want to become universal laws you can have a moral society.


So far to answer why I would be moral. But I do not deem this answer sufficient because yet I have not laid out what my maxims that I would will to become universal law are. For this I made this little drawing above. My moral actions are ruled by 3 principles: Humanism, utilitarianism and hedonism.
A moral action is an action that makes it to one of the levels that are shown in the sketch. Therefor if you remove the pillars of Humanism for example the action would not be moral at at. It could comply with the rules of utilitarianism or hedonism as much as possible, if Humanism would get violated I would not deem the action moral at all.
So what do those -isms mean:

Hedonism:
Hedonism judges the moral validity of an action by the pleasure it brings. If I go to my wardrobe and decide what shirt I should put on my choice would normally be ruled by the question what shirt I like this day most.
Platon's criticism of Hedonism was: "That it brings pleasure to scratch yourself when it itches, does not give any moral value to scabies."

therefor I need a second moral principle on which to ground Hedonism which is

Utilitarianism:
Utilitarianism judges the moral validity of an action by the overall utility of its outcome. Therefor I might get some short lived pleasure by getting myself infected with an athlete's foot and then scratching myself, but the overall outcome would be that negative that this action would at least not be advisable. If I return to my example with the wardrobe: When I want to go shopping by bicycle and I choose what I should wear during this, my choice would not just be governed by Hedonism but also by Utilitarianism: I would try to choose colors with which I can be easily seen. The question whether I would choose lime-green or bright yellow or a screaming red would again be a matter of taste and therefor Hedonism.

Humanism:
Humanism is the basis of my moral judging. And it has to be. Humanism as the basis forbids against seeing other humans as means to a purpose and not as purpose for all means. There are two axioms that describe very well what Humanism is:

1st: Everyone has the right to live.
2nd: Human Dignity is inviolable.

Humanism is necessary to explain why it is despicable that Stalin killed and mistreated millions of people for the sake of his revolution. On a purely Utilitarian basis this would have been tenable if the projected outcome would have just been good enough.
Just with Humanism you have a basis on which to condemn to make human beings means of your purposes.

The question that remains now and is often asked by theists is: How does an atheist base his or her Humanism.
There are two answers to this. Firstly if I do not want to be made a mean of someone else's purpose then I also can not make others means of my purposes. Secondly there is also historic experience (3rd Reich, Stalinist Communism) who have shown that in systems that lack Humanism the customs will brutalize very quickly.