In this thesis, I consider the consequences of applying Thomas Nagel's theory of the subjective and objective to rational decision-making in ethics. In chapter one, I am concerned with the notion of rationality. I discuss a common assumption man has about rationality and analyze the four necessary conditions for rationality. In chapter two, I present Nagel's theory. I draw out implications which are important in connection with rationality. In chapter three, I examine the implications of Nagel's theory for the notion of rationality. I focus on two problem cases which are designed to highlight a conflict between Nagel's theory and rationality. I use an analogy to Derek Parfit's work on prisoner's dilemmas to support my claim that the acceptance of Nagel's theory rules out the possibility of performing actions based on rational decisions. Finally, I argue that the type of rationality which is possible under Nagel's theory is deprived of any action-guiding force.