However, it must be understood that for any given light to carry out its function, its source must be strong and enduring. The light may then spread forth or not depending on what is required – but it cannot cease to exist at a certain time thus requiring the creation of the same light anew at some other time. In these terms, Abba and Imma are the very source and root of the Mental Powers of Zeir Anpin, and this root must not be damaged. However, the role of Israel Sabba and Tevunah is to bring the Mental Powers into Zeir Anpin. Since the Mental Powers in Zeir Anpin are subject to ascents and descents, the coupling of Israel Sabba and Tevunah is not constant.