The fact that the biosphere responds unpredictably to our actions is not an argument for inaction. It is, however, a powerful argument for caution, and for adopting a tentative attitude toward all we believe, and all we do. Unfortunately, our species has demonstrated a striking lack of caution in the past. It is hard to imagine that we will behave differently in the future. We think we know what we are doing. We have always thought so. We never seem to acknowledge that we have been wrong in the past, and so might be wrong in the future. Instead, each generation writes off earlier errors as the result of bad thinking by less able minds--and then confidently embarks on fresh errors of its own. We are one of only three species on our planet that can claim to be self-aware, yet self-delusion may be a more significant characteristic of our kind.