The Doubt of the Manas

The mind consciousness can think about rather abstract things, whereas the manas, because the dharmas it grasps at are so numerous, has a coarser discriminative wisdom and does not easily resonate with abstract thinking. He always wants to experience things personally, believing something to be real only when He has seen it with His own eyes; only then can He accept it. Therefore, if the manas still has doubt about something, He will keep it on His mind, with every thought fixed on that matter, and only after personally experiencing it will He let go of the doubt in His mind.

The saying "the heart won't die until the Yellow River is reached" refers precisely to the manas. If the manas has not reached the ultimate source of a matter, He will not give up. If He still has doubt about something, He will keep clinging to it, always wanting to investigate it until it is clear, until there is no way left to go—only after hitting the southern wall will He be willing to turn back.