The series often revolves around forbidden love. These relationships are not allowed for various reasons: some involve medicine cats, who are not allowed to have mates according to the medicine cat code, while others develop between cats in different Clans, which is also forbidden by the warrior code (for example, Graystripe of ThunderClan and Silverstream of RiverClan). Holmes said that another central theme of the series is "faith and spirituality" regarding StarClan. All books in the series feature the influence of StarClan, not just as the cats think of them, but in terms of prophecies delivered by StarClan which inevitably come true. Some scenes take place within StarClan's realm, with no living cats present. Thus, the existence of an afterlife and the influence of spirits who have passed on and yet retain their earthly identities is integral to all of the plot arcs in the series. Another idea explored in the novels is the reactions of different faiths when meeting each other. For example, the Tribe of Rushing Water, which believes in different spiritual ancestors than the Clans, is introduced in "Moonrise". In an author chat, Holmes explained that the books never say that either the Clans or the Tribe of Rushing Water is right about faith because both are "equally valid". This leads to fear and suspicion between them because they are afraid of things they do not understand. Holmes remarked that "ignorance is a very scary thing!" Non-belief is also explored in the storylines through characters, like Cloudtail and Mothwing, who do not believe in StarClan.