The crucial interpretive hypothesis of the present book is that central parts of Spinoza’s theoretical philosophy, and in particular his philosophy of mind established in Part Two of the Ethics, can only be understood against the backdrop of the conviction that subjective experience is explainable, and that its successful explanation is ethically relevant because it makes us wiser, freer, and happier. The introduction discusses, in a general manner, what requirements a philosophical system that aims do this must fulfill. Such a system, it is argued, commits itself to a realist version of extreme rationalism that maintains the difference between thought and reality; it cannot, as is sometimes assumed, turn into some form of absolute idealism. Furthermore, in looking at how this fits with Spinoza’s approach in the Ethics, the book shows how the structure of Part Two, along with Spinoza’s proceeding in geometrical manner, must be comprehended.