What does it mean to be human? What is the deﬁ nition of humanity? What is the measure of humanity? These are age-old questions with which philosophers have occupied themselves ever since they turned their gaze away from the natural world toward the human being itself. To say that these are questions with which philosophers have occupied themselves is not to say that they are merely theoretical questions. The Universal Declaration of Human Rights is one example of the way in which a particular deﬁ nition of what it means to be human has had wide-ranging practical ramiﬁ cations. The advancement of life sciences and life technologies in the twentieth century, which has made it possible to intervene in the creation and termination of human life in unprecedented ways, is another example which shows that questions about what it means to be human can literally be a matter of life and death.