The two traditional Indian Buddhist philosophers - the Mādhyamika Nāgārjuna (c.150–250) and the Sautrāntika-Vasubandhu (c. 350–430) - agree that mental causation involves a causal relationship between successive consciousness moments in which the previous moments are causes and the latter moments effects. In this chapter, I investigate the nature of this relation at stake. Is it a type of relationship that requires (1) necessary connection between successive consciousness moments in which there is an internal causal connection between the previous and the latter conscious moments? (2) Alternatively, is the causal relationship between consciousness moments just a matter of constant conjunctions of intrinsic natures (svabhāvas) without any necessary causal relation? If the former, the two Buddhist philosophers need to explain which of the two truths - conventional truth or ultimate truth – does this necessary relation belong and why? If the latter, they need to explain how mental causation – a continuum of interrupted mental events in which the previous moment are causes and the subsequent moments effects - possible with intrinsic natures, absent any necessary causal connection between them? Either way, the Buddhist philosophers must address these two questions concerning causal relation adequately.