Perhaps one of the greatest problems the atheist worldview faces is the issue of the natural versus the unnatural. All of their subjective basis for morality and action (epicurianism, survival, or whatever they select) is based on nature. Nature causes us to feel pain, and pain is a feeling most would choose to abstain from. Likewise, pleasure is a feeling that humans tend to enjoy, so pleasure is generally accepted as a good thing. Morality, for the atheist, is based upon these natural things. This may not seem like an outright problem, but considering that evolution of our species is defined as a change over time, one begins to see that what is valued or valuable today may not be the standards and morality of tomorrow.