This is, of course, a philosophical assertion, yet upheld by many different philosophies. The value and preservation of human life are central to Western thought. The argument can be made from autonomy (Locke), Natural law (Hobbes), and even, virtue ethics focused on vocation and common good of neighbor.
Freedom of speech, expression, and religion are always asserted and rarely argued. The constitution upholds a set of principles promised to every individual in this land that is not contingent on the Government nor the philosophy of those individuals. The sad thing is, those principles all essentially assert that all human life is valuable, yet our history shows that we have been more than willing to treat some people as sub-human.