In this paper we shall discuss the relativistic space-time metric. Even if all inertial frames of reference are treated as equivalent in the formalism of the theory of relativity, there is an important difference between them if we take possible observers into account. The class of possible frames of reference for human or man-made observers is a proper part of the class of conceivable frames of reference. This subclass is privileged with respect to human knowledge: Descriptions of physical phenomena with respect to members of it have a different epistemological status from descriptions with respect to other frames of reference. This fact is often neglected in presentations of the theory of relativity. We shall attempt to show that an emphasis on the observer's position sheds light on some features of the theory of relativity.