A lot of people say that morality can't be defined for everyone, or that it changes for everyone, or that there's no real way to know what's moral except for that feeling - "you just know." Still others say that it comes from some supernatural being that no one can prove exists, who's very existence rests on the fact that no one can prove it. There are a few people, though, who disagree with both of these views. I am one of them.
Morality, at it's core, is a code that ensures you live well on earth, that you're a good person. Here, too, there are disagreements on what a good person is, even though most virtues are universally agreed upon, and generally revolve around the golden rule idea - treat others the way you'd like to be treated. This doesn't always work, however - some people enjoy being hurt, but that doesn't make it right for them to hurt others.
The golden rule is a pragmatic code, not a principle. This is why it doesn't always work. It rests on the assumption that you will be treated the way you treat others, and while this is often true, it is not always true. What happens to the moral person who bases his morality on this code? He is honest, because he'd like others to tell him the truth; he is kind, because he doesn't like it when others are mean to him. But those others may lie and treat him badly even though he has treated them well. This may disillusion him, and then he has nothing left to base his morality on. Without a reason for morality, he may abandon it.
Morality based on religious doctrine is no better. Without proof of the existence of the dictator, men have no real reason to follow his rules, other than their emotions. In this case, when asked to explain to others why they should not kill, their reasons begin with "Because God commands it" and ends with "It's just wrong." Although religious doctrines give a semblance of reason for morality, there is nothing within them that can be applied to nonbelievers. Again, without a real reason for morality, a man may abandon it.
The principle behind both the golden rule and most religious doctrines is the equality of men. All men are born as men, and so should be treated as men; not because you want to be treated as one yourself, but because you are one, and treating others like animals instead of men means that you do not consider yourself a man. But what does it mean to treat men as men?
It means treating a man as if he has a mind, as if he has volition, as if he has the ability to produce. Every normal human is born with these aspects - they are fundamental characteristics, shared by no other species we know about. These are the things that men must use to survive, and pretending they don't exist means taking away a part of his livelihood, essentially killing a piece of him. Since these are aspects of all men, pretending they don't exist in others is the same as saying they don't exist in yourself, an act similar to suicide. You would essentially be saying "I can't live on my own merit." In cases like this, Darwin eventually proves his point and agrees with you - if not in body, then certainly in spirit. The spirit of those who do not think they are men is the spirit of the defeated, the destructive, the despairing. This is no way to live.
There are, of course, specific actions that involve treating men as men instead of as animals. For example, murder involves pretending that men have no volition, that they may not choose whether or not to live. Theft involves the same, as well as pretending that men have no ability to produce, since anything a man produces has been taken from him. Dishonesty means pretending a man has no mind, pretending that there is some aspect of reality he cannot bear or cannot understand. Morality also includes actions that only affect yourself - suicide, altruism, and self-deception are the equivalent acts.
I have said that pretending volition and intelligence don't exist in others is saying that you also don't have them. Why, though, should you treat any man as if he has these things, including yourself? This is the most basic question of all - why there should be any system of morality at all. Most men recognize that morality is a good thing, that it helps men get along and helps men treat each other well. But what is it that makes a specific action truly good, versus simply not evil?
It depends on what you consider the point of life to be. If the whole point of a life is the life itself - if, for example, a dog's purpose is to live the way a dog should - then the good is anything that furthers that goal. You cannot condemn a dog for chasing a rabbit because eating a rabbit would be right for the dog, although it wouldn't be good for the rabbit. Avoiding the dog is good for the rabbit. Similarly, what is good for a human is the action that allows him to live. Many religions agree with this one, saying that suicide is morally wrong, even though they do not say that life is good. Once your own life is the standard by which you judge your actions, morality follows easily.
Spoonful of the day: "A living entity that regarded its means of survival as evil, would not survive. A plant that struggled to mangle its roots, a bird that fought to break its wings would not remain for long in the existence they affronted. But the history of man has been a struggle to deny and to destroy his mind." ~ Ayn Rand, from For the New Intellectual.