If sustainability is considered on large temporal and spatial scales, then more humans can occupy the planet over the next century, or even millennium, than would be possible if the ecological life support system is pushed beyond its endurance. Anyone even partially literate in toxicology realizes that humans cannot kill every species on the planet without killing themselves. Indeed, disappearance of any species indicates unfavorable conditions for life in general, whether the conditions be toxicological or physical. Therefore, humans are inevitably affected by any condition that affects large numbers of other living creatures, and it is in enlightened, human self-interest to protect the natural systems upon whose services human society depends.

However, some people believe that technology can solve every ecological problem (even though the problems are often caused by technology), and others believe that human ingenuity, intelligence, and technology free humans from the harsh biophysical laws (natural laws) that restrict other species. This argument is difficult to counter because of remarkable advances in science and technology over the last 200 years. On the other hand, these advances have also created new problems that did not exist 200 years ago, which have not yet been solved (for example, safe longterm storage of radioactive waste). Present storage technologies may be `safe', but persuasive evidence indicates that the safety has not yet been adequately validated. Basically, this mind-set is a version of 'trust us', `the check is in the mail', and `the persons who created the problems can also solve them'.

Certainly, these options are not always true. Technology often advances faster than changes in social behavior evidenced by the fact that, in an era when weapons of mass destruction exist in incredible quantities, some governments and political leaders still act in ways that are generally viewed as irresponsible and self-serving. Reversing severe ecological damage in time frames of relevance to human society (that is, to avoid major suffering) is not possible, and only one major mistake is required to cause severe societal disequilibrium. One of the goals of sustainability is to avoid such an unfortunate situation, both for present and future generations.

Moral reasons exist for protecting natural systems and the organisms that inhabit them. Sustainability does not address these ethical issues, except indirectly to the degree of leaving a habitable planet for future generations. Further, humans hope that their descendants will have at least the same opportunities to enjoy natural systems that they had. Persons now living cannot imagine future circumstances or what the value system of future generations will be. Sustainability initiatives do not attempt to address these issues, but rather to leave ecological and societal capital in place for present and future generations so as not to restrict any options.
 

This contentious issue is probably best examined by repeating the old saying `your right to swing your fist wherever you choose ends where my nose begins'. A social contract diminishes the opportunities for an individual to do serious harm to other individuals in society while exercising freedom of choice. Members of society voluntarily relinquish certain individual rights clearly perceived as harmful so that their lives are not disrupted by inappropriate actions of others. Stated more crudely, individuals make a social contract to avoid damaging behavior with the understanding that society will coerce all of its members by imposing consequences for inappropriate behavior.

The situation becomes more complicated when private property is involved because people feel that they own the property and should be able to do whatever they wish with it. However, the same principle applies —a social contract requires that individual behavior must be restrained if such behavior endangers either the freedom or the property of others. Some residential areas even extend this principle to the aesthetic, where hanging laundry outdoors is considered inappropriate and antenna towers for ham radio operators are not permitted. A general, social contract is essential if one moves from one part of a country to another and wants to assume that certain protections are in place.

Sustainability merely modifies the social contract, i.e., general consent to protect future generations from the actions of present generations that will jeopardize their opportunities for living a comparable life. Sustainability, therefore, is not anti-private property but merely seeks, with common consent of society in general, to ensure that future generations have a habitable planet and at least the same opportunities as present generations.

The problem with all social contracts is that the perception of an individual action may be that it causes no harm —for example, filling in a one-acre wetland in a drainage basin that extends from the Canadian border to the Gulf of Mexico. But, filling in thousands of acres reduces storage of flood waters. The exhaust from a single automobile can hardly be measured any distance from the exhaust pipe. However, the exhausts from millions of automobiles have a dramatic effect within the limits of a city, especially when there is a temperature inversion or some other factor that keeps the air confined to the city as if it were covered with a dome. When an individual fights restraint either on individual behavior (such as speed limits) or on use of private property, the question is often not just whether a particular type of behavior is irresponsible. If only one person were acting this way, it might not be; however, the question becomes: what if everybody acted this irresponsibly? Sustainability is merely a social contract with large temporal and geographic spans, not different in principle from any other social contract.
 

The Brundtland Report (United Nations World Commission on Environment and Development, 1987) states that major societal changes will be essential in order to achieve sustainability. Rather than being anti-change, advocates of sustainability want to cease present activities that are not sustainable and change them to ones that are. Therefore, people against sustainability could be considered anti-change, and those for sustainability are pro-change (at least where non-sustainable activities and behaviors are concerned).
 

The evolutionary process is continually producing new forms, most of which are no better suited to present environments than existing organisms, but a notable few are. Sustainability is essentially an attempt to ensure favorable conditions for one species, Homo sapiens, over large temporal and spatial spans. Many other species compete with humans for a wide variety of resources; the most successful in the competition will have a greater fitness than other species. From a human perspective, this situation will lead to some destabilization. Sustainable development and its other descriptors acknowledge that humans are dependent on natural systems, but the question of how competition from other species is handled has not been addressed adequately. Certainly, not all interactions with other species will be benevolent and might even require drastic measures. This complex issue will probably prevent sustainable development from being the steady state perceived by most persons. In addition, general goals and conditions can endure if they are continually modified, but prescriptive rules and regulations for sustainability applied too bureaucratically will not suffice.

I am indebted to Eva Call for transcribing the dictation for this manuscript and to Darla Donald for editorial assistance. Larry Moore, Jeff Slack, Bruce Wallace, and B. R. Niederlehner furnished useful comments on the first draft.