 Few things in biology have been more extensively discussed than the definition of life. It is frustrating so little progress has been made on the topic in the face of so much research, theory, and debate. There are many reasons for this failure: disagreements about how abstract or specific definitions should be, different commitments as to what ought to be included in a definition, and even disagreement about the nature of definitions themselves. This section covers the nature and role of definitions. Each of these has been used in approaching the question of life.