  This account of the mental in terms of the irreducible units of  experience or dharmas is not without it problems (see  Piatigorsky 1984, 181; Waldron 2003, 52). If what we take to be mind  and mental phenomena are in fact discrete and momentary series of  cognitive events, how are we to account for those elements in the  mental stream that appear to subsist and continue? The dual use of  vijñāna in the early Abhidharma literature to  designate both a momentary discerning faculty and a persisting  sentience, simply masks the problematic: a descriptive account of  vijñāna in terms of a succession of momentary cognitive events, which alone are real and causally efficacious, cannot explain how past events outside the immediate causal series can affect the present. In its second use as pure consciousness or pure sensation, vijñāna takes on the role of explanans. The possibility that this secondary sense of vijñāna could be mistakenly taken as suggestive of an abiding, enduring reality, explains in part why the Abhidharma  psychological vocabulary includes terms such as pudgala  (‘individual’ or ‘person’) and  citta-santāna (‘mind-steam’).