Is there, or is there not, a definite set of traits setting humans apart from other sentient beings – and if so, what are those traits? This is a question that has vexed philosophers, poets, and other humanists ever since the time of the ancient civilizations.  It bears direct relevance to the debates over human enhancement because, if such a core set of characteristics exists, then any major alteration of that core through the re-engineering of its contents could potentially threaten our fundamental identity as a species.
This is precisely the position adopted by a conservative school of thinkers whose ideas are best articulated in the works of the scholars Leon Kass and Francis Fukuyama.  Since these social theorists were appointed by President George W. Bush to the President’s Council on Bioethics in 2001 (with Kass serving as chairperson), their arguments have been widely influential. Although both Kass and Fukuyama acknowledge the difficulty of pinning down any precise list of uniquely human traits, they nonetheless share a passionate belief that such a core does exist, and that it is under serious threat. Biotechnology, they argue, is gathering powers that will allow it to distort beyond recognition the essential features making us who we are. We therefore need to mobilize all the resources we can – legal, economic, cultural – to stop it from doing so.
Opponents of this position have summoned up the long list of eminent thinkers who, since the time of the Enlightenment, have concluded that there is no such thing as a fixed and definable human nature at all.  This is the tradition of the “blank slate” or tabula rasa, going back to John Locke: it holds that humans are born with few (if any) innate characteristics, and that most of our traits are generated, shaped, and honed through our experiences after birth. This Lockean tradition emphasizes the immense plasticity of human beings when they are born, and the powerful role that rearing, socialization, and education play in determining who we will become. 
Seen from the perspective of the blank slate, human enhancement is not necessarily problematic at all. Since there is no fixed “human nature” to start with, we run no danger of violating any putatively sacrosanct core of human identity. On the contrary, we humans have always possessed the option of making ourselves into most anything we choose. Therefore, if contemporary science and technology provide us with new tools with which to shape our own selfhood and that of our offspring, so much the better. The modification of human traits through chemical, bioelectronic, or genetic means is merely an extension of the potent social and cultural shaping processes that humans have been applying to themselves and their children all along.
As of the second decade of the 21st century, however, the theory of the blank slate lies in tatters. It has undergone steady assault from many academic disciplines, ranging from neuroscience to anthropology, from cognitive psychology to linguistics to behavioral genetics.  As the discussion of genetics in Chapter 4 shows, moreover, the ‘nature vs. nurture’ debate is now utterly obsolete: the only accurate way to understand the genesis of human traits is as a dynamic, ongoing process of reciprocal causation in which innate propensities interact continuously with environmental influences. This interactive shaping process commences with human conception and continues not only through the gestation and early development of a child, but throughout a human life, all the way to the moment of death.
Therefore, while it is certainly true that humans are highly malleable creatures, it is a structured malleability that characterizes us: we are not like silly putty, capable of being molded into any arbitrary shape. From the moment of conception, we are beings pre-endowed by our biology to develop certain broad faculties, capabilities, and propensities, which natural scientists and social scientists have come to identify with considerable specificity. Our malleability lies in the countless profound ways in which our experiences can shape and channel the development of these innately structured faculties over the course of our lives – and the staggering range of human phenotypic and cultural diversity is a testament to the enduring breadth of our pliability as a species.
We thus find ourselves in a conceptual middle ground. The accumulating evidence from the natural and social sciences suggests that we need to reject the one-sided explanatory frameworks put forth by both poles of the ‘nature vs. nurture’ debate. We are not blank slates, but neither are we preprogrammed automata. It is just as silly to say that we are pure unformed potential at the moment of our birth, as it is to say that our adult character, intelligence, or behavioral traits are pre-ordained from the start by genetic shaping powers. The reality is much more complex (and interesting) than that: our lifetimes are an endless formative give-and-take process through which innate conditioning factors and our social and personal experiences continually constrain and potentiate each other. Over the course of our lives we can become (and make ourselves into) a great many things, but we cannot become absolutely anything we dream up. There are limits, and those limits constitute our human nature.
 See Steven Pinker, The Blank Slate: The Modern Denial of Human Nature (Viking, 2002); Christian Smith, What is a Person? Rethinking Humanity, Social Life, and the Moral Good from the Person Up (U. of Chicago Press, 2010); PeterLoptson, Theories of Human Nature (Broadview, 2006); Louis Pojman, Who Are We? Theories of Human Nature (Oxford, 2006); Leslie Stevenson and David Haberman, Ten Theories of Human Nature fifth ed. (Oxford, 2009).
Leon Kass, ed., Beyond Therapy: Biotechnology and the Pursuit of Happiness. A Report by the President’s Council on Bioethics (Regan Books, 2003); Leon Kass, ed., Being Human: Core Readings in the Humanities (Norton, 2004); Francis Fukuyama, Our Posthuman Future: Consequences of the Biotechnology Revolution (Farrar, Straus, 2002).
The most comprehensive overview of this research is given by Pinker in The Blank Slate.