What is the co-existing support of mind consciousness?

Sahabhū-rati (simultaneously existing basis) means that two or more dharmas exist together—they are already together at the moment of birth; where there is this, there is that, and this dharma depends on that dharma in order to exist. If the five faculties were the simultaneously existing basis of the mind consciousness, then the five faculties would be a necessary condition for the birth of the mind consciousness: without the five faculties, there would be no mind consciousness, and wherever there is mind consciousness, the five faculties must be present. 

But is this actually the case? In terms of the forms of sentient beings, many sentient beings do not have the five faculties, yet they still possess mind consciousness. For example, the devas of the formless realm, or bodhisattvas and great arhats who manifest without bodies, and so on—they all have mind consciousness but no five faculties. The solitary consciousness (mano-vijñāna-eka-citta) of all sentient beings does not depend on the five faculties for its birth and existence; when the mind consciousness leaves the body in profound meditative concentration (dhyāna), it can still discern, without needing the five faculties. Therefore, the five faculties are not the simultaneously existing basis of the mind consciousness. The simultaneously existing basis of the mind consciousness is the eighth consciousness, the manas, and the seeds. 

The mind consciousness requires very few conditions for its birth, so it arises easily and does not easily cease. The five consciousnesses all cease and no longer exist in meditative concentration at the second dhyāna and above, whereas the mind consciousness only ceases in the concentration of no-perception (asaṃjñi-samāpatti) and in the cessation of perception and feeling (nirodha-samāpatti). Therefore, the simultaneously existing basis of the mind consciousness is neither the five faculties nor the five consciousnesses.