Theists often like to make the claim that their god is the basis for morality, despite the fact that the abrahamic god drowns almost all the inhabitants of planet earth because he was angry at them. I'm not going to dwell on the atrocities of the abrahamic god because it is like shooting fish in a barrell... with a missile.
Rather I want to consider how morality can be calculated reasonably by anyone with a brain. Humans can think about their own morality, so even if we had no moral instincts, we could figure out the difference between right and wrong by thinking of what we want. Consider the following scenario:
My neighbour has uprooted my beautiful tree while I was out because it was littering leaves into his garden. My garden looks bare and pathetic, and the ruined tree lay there next to a huge hole. My first thought is that I want to kill my neighbour. I am blind with rage. Let's assume there is no such thing as a state or a law code that is enforced by a leader. An anarchistic society.
If I kill my neighbour, his family might suspect me, and enact a revenge killing. They might kill me, or if they really want to they can kill someone close to me, something which is somewhat worse than death. If I consider the action of killing him there is a direct reason for me not to do it. This kind of moral reasoning pervades human morality. People shoplift more if they know that they can get away with it. Societies with ineffective justice systems also have high levels of crime. Deterrence is a powerful moral reasoning tool used by human beings.
A good point to raise is that such a moral system would indicate that it is okay to do something unless you get caught. In other words, the logical implication of such a system is that Consequence dictates course of action.
That just won't do. Everyone ultimately chooses what they consider moral, but there has to be some rules that are common to humanity. Another system makes sense in this regard. In a society of organisms if your morality allows the suffering of others there is no reason for them not to allow your suffering. In other words a truce where each party agrees not to let the others suffer prevents future suffering by oneself. But you might say that under such a moral system we would fail to protect minorities, because there is no point in a truce with a weak opponent. Although this is somewhat rational it is detrimental in the long term. Society changes, and it may be that one day you may find yourself or someone you care about part of a minority that is abused and oppressed. Moral rationales have to work on a long term and sustainable basis. Nobody could reasonably want to suffer in the future, so that must always be considered a possibility.
This brings me to modern moral philosophy. Namely when we think of rules, we should be ignorant to whom those rules will apply to. In other words this encourages the type of long term thought required to consider your future self or others you care for. If you hold no unpopular opinions currently, that does not mean that society cannot change. Constitutional documents and Bills of rights codify such rules, so that if we find ourselves judging a segment of society we treat them no more harshly than they are allowed to treat us, if the tables have turned. This is different from fear of reprisals, because the oppressor does not necessarily become the new aggressor. Any aggressor would do. It's all about treating everyone equally.
What is lacking in this essay is a definition of good and lists of virtues and vices, because those kinds of things are infinitely hard to figure out from our subjective points of view. There are people that enjoy things we might find repulsive, be it blue cheese, pornography or justin bieber. People can think of reasons why these things need to be outlawed, because each person rationalises their own views of what is right and good and pleasing. The problem is when morals invade preferences. With the above principle in place, we could reasonably ask how we should determine whether something is bad by imagining whether it would be bad if it was applied to us. For instance state religion is a bad idea because if a religion other than your own is imposed later, it will be detrimental to you.
The classical liberal view is that things that don't harm others should be up to the individual. No matter how much you argue, one man having sex with another man does not directly affect you, neither does justin bieber. That way we can be sure that things aren't merely branded immoral because someone dislikes them, but because they cause real harm.This system of reasoning is not only sound, it is the system of moral reasoning that governs modern liberal societies. The success of these societies owes a lot to these methods of moral thinking, and this represents the cumulative work of many thinkers. This leaves a universal concept of what is good out, strictly so that people can decide what they deem good, without harming others. This is a moral principle that forms one of the cornerstones of liberalism.
So what is this theistic objective morality that is sometimes referred to? I think the idea is that actions can be good in themselves, that there are some intrinsically good actions and bad actions. This fails terribly when you consider the various corner cases that moral choices can offer for which there are no obvious answers. Is it moral to kill one innocent person to save two? Ten innocent people to save 20? One thousand to save one thousand and one? That kind of thing brings us into the realm of interesting moral questions. Would it be moral to kill a baby if you knew that baby would later become Hitler?
So the rational basis for morality is basically based on our fear for repercussions, our need to avoid suffering and our need not to be harassed by others if we are not harming them. None of this is new, and I am basically just rehashing some very basic moral philosophy. I know there are many subtle points that can be brought up and that would make for interesting discussion too, but I wanted to give a basic introduction to the idea that secular morality does not require any gods, but that objective moral values can still exist, such as that we shouldn't harm others.Morality is ultimately a type of social glue. If members of a social species mistreated each other constantly and failed to cooperate there would be no point in them being social in the first place. Being social requires some sort of advantage to being social. That is an interesting topic with its own lines of evidence, but it should be mentioned here because there is no such thing as intrinsic good for its own sake. All morals are a means to the end of the advantage of social cooperation.