What's the word that means "an extra one in case it breaks"

Which is the best summary of emerson’s view of solitude expressed in society and solitude? spending time in solitude is preferable to spending time in society, and it is ultimately more beneficial. though solitude is not necessarily valuable in and of itself, it is important to recognize its significance within a larger community. solitude can be beneficial in that it allows the mind to contemplate necessary and difficult questions. only through spending time in solitude and in deep observation of the natural world can one find happiness within society.