The goal of this dissertation is to have a better understanding of three aesthetic theories that I take to be central in western aesthetics since the modern period, i.e., aesthetic formalism, the theory of aesthetic supervenience, and modern aesthetics, by examining them with a perspective that I derive from the debate between externalism and individualism in the philosophy of mind. I argue that the three aesthetic theories under examination can be seen, firstly, as centrally concerned with psychological issues, and secondly, as based on the individualistic picture of the mind. And since individualism as a picture of the mind has already been shown to be the basis of the major theories of mind, knowledge, and meaning, starting from Cartesianism to theories of quite recent years, we can say that these aesthetic theories were also shaped by the philosophical current in which psychological individualism was deeply ingrained and powerfully operative. This also reveals that these aesthetic theories are in conflict with psychological externalism, which has been widely acknowledged to do justice to ways in which we characterize the content of a mental state. This conflict leads me to question why psychological individualism has been consistently used in these aesthetic theories without being noticed or critically discussed. To be precise, there is an individualistic assumption which is shared by aesthetic formalism and the theory of aesthetic supervenience and is developed from modern aesthetics, and the fact that this assumption has not been noticed by others suggests to me that there is something intuitive in the assumption. With this point in mind, I explore what intuition is behind the assumption and show that the intuition playing a crucial role in the Cartesian skeptical thought experiment and also in the standard theories of perception is essential to these aesthetic theories.