The rupa and arupa skandhas

We find it very difficult to come to terms with this frequently heard contention that the arupa skandhas or kandhas are in some way or another capable of carrying out things by themselves, such as initiating and maintaining an "incessant interaction" or making something "enter or realize Nirvana". Because the arupa skandhas in fact do nothing - they are the doing. The cluster of physical existence is the rupa skandha. Also this cluster does nothing - it merely is physical existence in all its aspects. The traditional four non-physical skandhas [sensations or feelings (vedana), perception (samjña, sañña), mental forces or formations (samskara, sankhara), and consciousness (vijñana, viññana)] are clusters or aggregates of functions, which are events - they denote how the rupa skandha is or becomes. Also the rupa skandha does not cause these events, it simply is them. Like when we say that a tree grows. The tree does not do the growing; it is the growing. This is how the tree is, how it exists in space and time. The growing of the tree is quite obviously an event, and not a thing, let alone a separate thing capable of in turn doing other things by itself. (from the Advayavada Buddhism website)