Twentieth-century science thrived on a strict fact-value distinction. Facts are one thing, we say, and values another. Science tells us the facts, but what we value is merely a matter of opinion or judgment. Values are relative to our culture—or, even more narrowly, to our subjective tastes. There is nothing objective in value. So we say.

But fact and value have not always been so widely separated. Aristotle held that each living thing has a telos, or purpose, an ideal state toward which it strives. The acorn strives to become an oak tree, the caterpillar a butterfly. But realizing one's telos means more than just maturing. The ideal state of an organism is full functioning, not merely maturity, but flourishing health.

In this Aristotelian view, fact and value are not so easily distinguished. It is a fact that organisms strive for health, that health is an ideal. But an ideal is a value. And so at least some values are also facts. On an Aristotelian conception it is factually better for an organism be healthy than unhealthy. Criteria of better and worse are thus built into the structure of the world.

The obvious objection to this line of thinking is that some organisms are pretty nasty, so it's not easy for us to regard their health as a good thing. Think of mosquitoes, for example, or heartworms—or any of innumerable hideous predators, parasites and germs. Such creatures can be healthy only by diminishing the health of others. Nature on the whole is therefore no peaceable kingdom. So while we can make some factual sense of the values healthy and unhealthy, better and worse, for specific organisms, there seems to be no factual basis for any more general or universal notion of goodness. Many people, of course, would like to believe that a beneficent and intelligent God rules the universe and establishes standards of good and bad. But no basis for such a hope is discernible in the physical or biological facts. On the contrary: Darwin did much to extinguish this hope by explaining how the telos of individual organisms can arise from the violent, random and globally purposeless process of natural selection.

Yet despite Darwin, the contemporary science of ecology provides reason to believe that value in nature is not relative merely to the telos of individual organisms. Life, viewed in ecological perspective, is hierarchically organized on a variety of levels: cells, organs, organisms, ecosystems and the biosphere. The concept of health is applicable on each of these levels. A healthy organ—a heart, for example—is better as a heart than an unhealthy one. Likewise, a healthy ecosystem is better as an ecosystem than an unhealthy one. This implies that values are not only built factually into the telos of individual organisms; they pervade the system of life at all levels.

To say that values pervade nature is not, of course, to show that humans should respect those values. We have never valued the health of tapeworms or mosquitoes and never will. But these organisms are special cases, for their health directly threatens ours. It is because of the conflict, because we rightly value human health most highly, that we disvalue the health of these parasites.

With ecosystems, however, and with the biosphere generally, there is no such conflict. Human flourishing is compatible with, and in many ways dependent upon, the health of the larger biotic community. This is not just a matter of taste or opinion, not just somebody's subjective judgment, but a matter of fact, of the structure and organization of life itself. Without fresh air we suffocate; without the photosynthetic activity of plants, we starve. Our health requires a healthy biosphere and our health is, as a matter of fact, a value for us.

Humans are, of course, free to refuse to value even their own health. Many do. Witness our consumption of cigarettes and junk food and our degradation of the biosphere. Yet not to value one's own health is ultimately self-destructive—and that, too, is not merely a matter of opinion but a matter of fact.