What is sustainability and what influences it?

I was asked to speak on the subject of ``What do we mean by sustainability and what factors have the most impact on it?`` I am afraid I must admit that if there is a generally accepted definition of sustainability, at least as applied to mankind and terrestrial systems, then I am not aware of it. But I expect you know that, or you would not be having this session. The subject of sustainability is one that interests us all. In fact, I think that it fascinates us all because the concept is one to which we can intimately relate. Sustainability implies eternal life; and throughout history, humankind has sought, sometimes secretly, sometimes openly, eternal life, if not for ourselves at least for our progeny. What I will try to do is (1) briefly discuss some of what has been said about this topic, (2) propose a definition that I believe is workable, and point out some of the issues that derive from it, and then (3) suggest what I think we can all do to ensure the sustainability of the planet. I confess to you at the outset that I am an optimist on this subject, and a technological optimist at that. By that I mean that I believe that humans and technology are inextricably linked and that technology provides one of the great keys to helping human beings achieve their goals. I also believe that sustainability is an achievable goal and that it is not a concept to be feared, either intrinsically or economically