Let us think for one moment about the important theme of memory in this book, which is of course a prevalent concern, as indicated by both the whole idea of visiting a place that you knew from your past, and the subtitle, which explicitly relates the novel to the memories of Charles Ryder. Above all, this book is a walk down memory lane as Charles revisits Brideshead mansion which triggers off the flashback that makes up the central part of this novel.
Part of this theme is the recognition that the past for Charles is an idyllic place. This is signalled by the title given to Book One, which is "Et In Arcadia Ego," which refers to Arcadia, a symbol for the golden age of pastoral romance and simplicity. Accordingly, the first book tells of a similar moment in the life of Charles when he first met Sebastian and of the wonderful time they had.
Again and again, almost becoming a constant litany, Charles compares how things were in the past to how they are today, and finds the present constantly wanting. For example, in the Prologue, Charles rants on about "Young England" and its various deficiencies. The nostalgia presented in this book takes the form of stylised memories that are clearly embellished by the imagination of Charles. Consider the romanticised nature of the relationship between Charles and Sebastian and the way that this time period includes such comments as the following:
I, at any rate, believed myself to be very near heaven, during those languid days at Brideshead.
The sense of nostalgia, but also the way that time is marching by and their youth will fade so quickly, is something that Sebastian comments on too, for he says: "If it could only be like this always--always summer, always alone, the fruit always ripe." Memory and nostalgia are therefore very important concerns of this novel, and let us remember that this is a work that focuses much more on the past than on the present.