Words are combinations of letters or rather syllables (varna). But how can we understand words and sentences when each syllable vanishes the moment it is uttered? The solution can be found either in our intellect or in sabda itself. For Prabhakara, it is found in the latter. Varnas have each an (eternal) potency of leaving a semantic impression which, combined with that of other varnas brings about the meaning of words and sentences.... (De Smet, Guidelines p. 219)Sankara was dissatisfied with this solution
because it is too particular to the understanding of a series of syllables. We should have a general solution which would explain our understanding of other kinds of series too (line of ants, of trees, of soldiers, etc.) Besides, in any case the impressions left by the varnas must be synthesized by the intellect. Hence, the solution must be found in the intellect. In fact, it is simpler to admit that our buddhi functions as intellectual memory, i.e., has the power of synthesizing elements apprehended at successive moments of time. This function is samasta-praty-avamarsini buddhi (intellect looking back on past experiences as a whole). Cf. Brahma-sutra-bhasya, 1,3,28. (De Smet, Guidelines p. 219)Yet more evidence for the active nature of intellect in Sankara.