Themes are simply ideas that Eliot develops in the course of the novel. It should be remembered, however, that what a good novel says is not detachable from the way it says it. The meaning is a part of the style and structure, and themes cannot be set out in so many pointed quotations. Meanings and attitudes are expressed through the whole work of art, and they must be studied as a part of it.
The major theme of Silas Marner is of course the influence of "pure, natural human relationships," but there are several others. Some of these are never the subject of a direct statement, but constant repetition brings them to the reader's attention, and the novel draws some sort of conclusion about them. One of these themes is the function of religion in society. Another is the use of custom and tradition. There is a more direct consideration, focused on Nancy, of the extent to which "principle" should predominate over sympathy in human relationships. This is closely connected to the question of indulgence versus discipline in human life, as exemplified by the home life of Godfrey and of Nancy.
A theme may be mentioned only indirectly and yet be quite explicit in its meaning. One such in Silas Marner is the effect of industrialization on English society in the nineteenth century. Lantern Yard after the factory has been built is a grimy, dark place full of unhealthy people. There is a sharp contrast between the grim unfriendliness of Lantern Yard and the community spirit of Raveloe, between Silas' life as a spinning insect and the fresh air of the open fields.