Theists usually believe that God is the author of morality, and that actions are only good or bad to the extent that God makes them so. A typical atheist rebuttal involves a reference to the Euthyphro dilemma: morality based on the absolute say-so of a supreme being seems to be no less arbitrary than the relativistic morality that theists decry.
This leaves open the question: how should unbelievers behave? Where does our understanding of morality ultimately come from? How do we know that our standards are correct in a meaningful and universal sense?
Although there is no such thing as unanimous agreement on complex philosophical issues, if we approach the question from a humanistic, scientific stand point, atheists ought to agree that there should be rational standards for arriving at moral conclusions. Like science and mathematics, useful systems of morality derive from some basic axioms, or recognize assumptions.
A few possible axioms in morality are:
- Every person has their own feelings and desires, and they are more or less similar since they are based on the same brain chemistry.
- When I look inward to my own desires, I fundamentally desire to pursue happiness and avoid pain and suffering.
- Other people have these same basic desires, and these desires are valuable to them.
- With all else being equal, it is better for people to be happy than not be happy.
- Conflicts arise mainly because people's desire to be happy and avoid suffering conflict with each other. The goal of secular morality is to resolve those conflicts in the best possible way for all concerned.
A few natural consequences of these axioms:
- All else being equal, it is wrong to needlessly inflict suffering on people.
- Except for the case of self-preservation, with all else being equal, it is best to avoid killing other people (on the assumption that they don't want to be killed).
- Actions such as slavery and rape are wrong because they excessively limit people's happiness and freedom of action.
Society is a construct that has arisen out of the need to enforce "good" behavior.
Game theory is a branch of mathematics that concerns itself with interactions between agents, with gains and losses, loosely called "games". To the extent that these mathematical models match reality, they can provide a mathematical underpinning for "moral" behavior, independent of the decrees of any god.
One interesting result of game theory is that the optimal strategy for a given player depends on whether the "game" is a one-time interaction or part of a longer series.
For example, assume that player A agrees to buy an object, say a book, from player B. Since A and B live in different cities, so A agrees to send a check for $50 to B, and B agrees to send the book to A.
At first glance, it would seem that A's optimal strategy is not to send a check: that way, if B sends him a book, he will have received the book for free; if B doesn't send him the book, he hasn't lost anything. Whereas if he sends B the check and B doesn't send A the book, A has lost $50. By a similar reasoning, B's best strategy is not to send the book, and hope that A sends him a check. If both A and B reason this way, they neither gain nor lose anything. (All moderately advanced societies have laws and law enforcement systems to prevent or reduce this type of thing.)
If, on the other hand, this is a repeating situation -- say, if A regularly shops at B's book store -- then the optimal strategy changes: the players remember the times in the past when they were cheated, and can anticipate that if they cheat this time, the other player will remember it next time. In this case, the optimal strategy is for both to play by the rules each time: A gains a book each time (and loses $50), and B gains $50 each time (and loses a book).
Biological basis for morality
One school of thought offers the notion that our genetic predisposition to survive as individuals has gradually shifted toward a desire to survive (and thrive) as a species. Our increased intelligence and understanding allows us to expand on simpler versions of social order that we witness in less-intelligent animals. In addition to utilizing our intelligence to analyze the outcomes of actions to determine the best outcome from a practical view, we are also empathetic beings who organize into complex social structures that range from simple families to nations, and beyond.
This claim that morality is a natural result of our intelligence and empathy is hardly new. The following quote from Charles Darwin expresses this concept clearly:
"Any animal whatever, endowed with well-marked social instincts, the parental and filial affections being here included, would inevitably acquire a moral sense or conscience, as soon as its intellectual powers had become as well developed, or nearly as well developed, as in man."
- — Charles Darwin
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Unfortunately Christian stereotyping sometimes influences impressionable atheists and they assume that because they are atheists they can’t be moral or can’t be very moral. Youngsters whose characters have not fully developed yet are most vulnerable to negative stereotyping of this type. Liberal Christians rarely or never do this but Christian fundamentalists stereotype atheists far too often. One particularly nasty technique that Christian ejvangelists sometimes use is deliberately provoking atheists or perhaps others they want to convert into doing something wrong/many wrong things. Then they say something like, "There! See how bad you are! You need Jesus!" If conversion attempts are successful probably the wrong things the person did/was provoked into doing before conversion will be regularly used to make that person feel guilty and inferior so the leaders of the group can keep the sinner under control.
If you are a teen
If you are a teen don’t let Christians convince you that you aren’t moral. You can be better than they are. You won’t set out to damage the confidence other people have and convince them that they can't be moral. You know that’s seriously wrong. You won’t imagine Jesus wants you to damage other peoples’ confidence that they can be moral.
- Humanist morality Atheism wiki on Humanist morality