Apas, water, is traditionally described as having taste for its sense and flavour, rasa, for its subtle quality, and this entry is the organ-independent, tanmatra-side statement of the same classical correspondence: rasa is what apas carries as its own distinctive contribution once shabda, sparsha and rupa have already been inherited from akasha, vayu and tejas in turn. By the fourth stage of grossening, apas is a genuinely rich element in Samkhya's terms, audible, tactile, visible and now tasteable, one stage short of prithvi's full complement of five.
This progressive accumulation is why water occupies the position it does, both in Samkhya cosmology and in a great deal of later Indian thought that inherits the scheme without necessarily inheriting its full philosophical apparatus: water is treated as more richly sensory, in a specific and countable sense, than fire, air or space, and less richly sensory only than earth. The ranking is not aesthetic judgment; it follows mechanically from counting how many tanmatras each element has accumulated by the point in the sequence it appears.
Rasa's corresponding organ, rasana, and its position fourth among both the Jnanendriyas and the tanmatras, complete a pattern that by this point in the twenty-five-tattva enumeration should be entirely predictable: one tanmatra, one element, one organ, added in a fixed order, each element carrying everything before it plus one new contribution, until the sequence reaches its final and grossest term.