Atheists and other free-thinkers frequently get accused of being “moral relativists.” Most religionists seem to equate moral relativism with immorality, or the complete lack of any morals. Unfortunately, some free thinkers have a tendency to come across as though they don't believe in morals in their attempts to explain what their values are.
Clearly the mere fact that there isn't some supernatural being out there dictating our behavior does not negate the fact that there are natural laws at work in our world. We understand that there are physical laws at work. We see that if you drop something, it falls rather than flying up in the sky. We know that water becomes a solid at a certain temperature.
There are also laws that are just as true and inescapable which govern our personal interactions. Personal integrity is an important principle. Recognizing the inherent worth and dignity of every person is another. We know that if we do not behave in a manner consistent with our professed beliefs, in other words if we do not show personal integrity, then people will not trust us or believe in us.
Note that I didn't say, “Lying is wrong.” What I said was that we must act with integrity. Occasionally, the best way to show integrity is by withholding the truth, or in rare cases even telling an out and out lie. For instance, during World War II, many courageous people sheltered and hid jews from the Nazis. Noone would say that they were damaging their integrity by lying to the Nazis and protecting their jewish friends. The principle of integrity was not violated by that. In fact, they were responding to a higher level of integrity. They were respecting the worth and dignity of the people they were protecting.
The same sort of “relativism” exists among natural laws. We know, for instance, that water freezes at around 32F (0C). If the water is placed in a high pressure chamber, however, it will solidify at a higher temperature. Under lower pressure conditions, it will take a much lower temperature for solidity to occur.
Just because there are variables in the rules doesn't mean we don't have to KNOW and LIVE BY the rules. In fact, the variable effects make learning the rules that much more important. If you're doing experiments using H2O, you really want to know what state it's going to exist in under the conditions you plan to create. It isn't enough after a while to rely on simplistic models.
So, we understand that there are universal, unchanging principles. We also understand that the way they are expressed and furthered by our behaviors can and will vary according to circumstance. We make a distinction between behaviors and principles. It's a more complex way of living, not an easy way out. We don't have someone standing over us to dictate our behaviors. We must consider and think and investigate in order to decide which behaviors will better fit the principles involved. Often, we have to identify the principles first.
Although it would be easier just to follow some pre-written rules by rote, it is more fulfilling to do the right thing because you know it's the right thing, because you've considered the alternatives and examined the principles involved and looked ahead to the possible consequences and made a decision taking all those things into account.