In the space of possible modes of being, the ones accessible to human beings form a tiny subset. Our biological constraints impose real limitations on what thoughts we can think, what emotions and enjoyment we can experience, and how long we can remain healthy and alive.
Just as much of the richness of human life and human relationships is foreclosed to the comprehension of even the smartest chimpanzee, so too there are possible values that lie beyond our own comprehension - this is, at least, seems like a modest and plausible conjecture. These values are currently unrealizable. If and when we learn how to develop new capacities and extend the ones we have, we might be able to access these wider regions of modes of being, and perhaps discover some that are fantastically desirable.
To significantly modify our biological constraints, we will need to use technology. Many of the requisite technologies can be foreseen, but we do not know how long it will take to develop them.
Posthumanism (or transhumanism to use the standard term) is the view that we ought to try to develop - in ways that are safe and ethical - technological means that will enable the exploration of the posthuman realm of possible modes of being. Transhumanists believe that all people should have access to such technologies. The choice of whether to use them, however, should normally rest with the individual.
The word "posthumanism" has also been used in other senses, for example to refer to a critique of humanism, emphasizing a change in our understanding of the self and its relations to the natural world, society, and human artifacts. Transhumanism, by contrast, advocates not so much a change in how we think of ourselves, but rather a vision of how we might concretely use technology and other means to change what we are - not to replace ourselves with something else, but to realize our potential to become something more than we currently are. Just as a child grows up and develops the capacities of an adult, new technological options might one day allow adults to continue to develop and to mature into beings with posthuman capacities.
The human species is still young on this planet, and it is possible that we have as yet seen little of what is possible for us to become. But success in this enterprise is far from assured, because we still have only our rather limited human wisdom and compassion to guide us through the transition. To develop greater practical and moral understanding would seem to be a first priority. This, along with development of human enhancement tools, efforts to reduce catastrophic risks, and work to alleviate the more immediate sources of human suffering, is enough the fill the days of responsible transhumanists and others who strive to improve the human condition.
Recounts the Tale of a most vicious Dragon that ate thousands of people every day, and of the actions that the King, the People, and an assembly of Dragonologists took with respect thereof. [J Med Ethics, 2005, Vol. 31, No. 5, pp 273-277] [Also in Hebrew, Finnish, Spanish, French, Slovenian] [html | pdf]
Wonderful ways of being may be located in the "posthuman realm", but we can't reach them. If we enhance ourselves using technology, however, we can go out there and realize these values. This paper sketches a transhumanist axiology. [Preprint, Ethical Issues for the 21st Century, ed. Frederick Adams, Philosophical Documentation Center Press, 2003; reprinted in Review of Contemporary Philosophy, 2005, Vol. 4, May] [html | pdf]