Aristotle is the first to argue that the task of natural science is essentially to investigate into the various causal relations that govern the processes of nature. This project deals with different questions, epistemological as well as metaphysical, that are raised by Aristotle's theory of causality. What kind of object is a cause: is it a thing or an event? Are causes 'out there' in the world, as objective features of this world, or are they rather to be thought of as elements in our explanations of nature? By exploring Aristotle's ways of dealing with and responding to this and other related issues, the project aims to contribute to our understanding of what it all means to explain nature in causal terms, as well as of what kind of knowledge such explanations may give us.