Rasa, Guna, Virya, Vipaka Explained
Every substance in Ayurveda, whether food, herb, or mineral, is known through five properties: rasa, the taste felt on the tongue; guna, the qualities it carries; virya, its heating or cooling potency; vipaka, the taste it takes on after digestion; and prabhava, the specific action that none of the other four explain. Together these are the dravyaguna, the pharmacology of the tradition, and they are the reason an Ayurvedic physician can reason about a plant never tasted before. The Charaka Samhita lays the system out in its discussion of taste and substance, and the Sharngadhara Samhita and Bhavaprakasha apply it plant by plant.
Rasa, the six tastes
There are six tastes, each born of two of the five elements. Madhura, sweet, is earth and water; amla, sour, is earth and fire; lavana, salty, is water and fire; katu, pungent, is fire and air; tikta, bitter, is air and space; kashaya, astringent, is air and earth. The elements explain the action. Sweet, sour, and salty are heavy and moist, so they pacify Vata and build Kapha. Pungent, bitter, and astringent are light and dry, so they pacify Kapha and aggravate Vata. Sweet, bitter, and astringent are cooling and pacify Pitta; pungent, sour, and salty are heating and aggravate it.
Rasa is the first thing known about a substance and the least reliable, because taste is only the beginning of what a plant does. Yet it is the basis of diet: a full meal contains all six, in an order that begins with sweet and ends with bitter and astringent, so that agni, the digestive fire, is neither smothered nor left hungry.
Guna, the twenty qualities
The gurvadi gunas are ten pairs of opposites: heavy and light, cold and hot, unctuous and dry, dull and sharp, stable and mobile, soft and hard, clear and slimy, smooth and rough, subtle and gross, dense and liquid. Each dosha is a bundle of these. Vata is dry, light, cold, rough, subtle, and mobile; Pitta is hot, sharp, light, liquid, slightly unctuous, and spreading; Kapha is heavy, cold, unctuous, slow, stable, soft, dense, and smooth. The whole logic of treatment is that like increases like and opposite reduces. A dry, light, cold substance raises Vata; an unctuous, heavy, warm one lowers it. Guna is how a physician reads a food or a season as well as a herb.
Virya, the potency
Virya is the substance's active power, and the tradition settles on two: ushna, heating, and sheeta, cooling. An older eight-fold scheme adds heavy, light, unctuous, dry, soft, and sharp, but the two-fold reading is the one the Ashtanga Hridaya teaches and the one used in practice. Heating substances increase Pitta, stimulate agni, thin Kapha, and calm Vata; cooling ones soothe Pitta, build tissue, and steady the mind. Virya is inferred from taste in the ordinary case, and named separately when it departs from it. Amalaki, Phyllanthus emblica, is sour to the tongue yet cooling, which is why it is the one sour fruit given freely to Pitta.
Vipaka, the taste after digestion
Vipaka is what a substance becomes once agni has transformed it, and it governs its long action on the tissues and the wastes. The rule is that sweet and salty tastes yield a sweet vipaka, sour yields sour, and pungent, bitter, and astringent yield a pungent vipaka. Sweet vipaka builds tissue and moves the bowels; sour vipaka increases Pitta and the acidity of the gut; pungent vipaka dries, lightens, and binds the stool. The exceptions matter as much as the rule. Pippali, Piper longum, and Shunthi, dried ginger, are pungent on the tongue but sweet in vipaka, which is why they warm without depleting and can be given where Maricha, black pepper, whose vipaka is pungent, cannot.
Prabhava, the specific action
When two substances share rasa, guna, virya, and vipaka and yet act differently, the difference is prabhava. Chitraka, Plumbago zeylanica, and Danti, Baliospermum montanum, are both pungent and heating, yet Danti purges and Chitraka does not. Brahmi, Bacopa monnieri, is medhya, an intellect-promoting plant, by prabhava rather than by its bitter astringent taste. Gems, mantras, and the metal preparations of Rasa Shastra act largely by prabhava. Where the four calculable properties and prabhava disagree, prabhava wins; where virya and vipaka disagree with rasa, virya wins, then vipaka, then rasa. The tradition knows a plant by what it does, and the properties are its account of why.
How the five work together
A physician uses all five at once. Facing a Kapha condition with cold, heavy, sluggish digestion, the choice is a substance that is pungent or bitter in rasa, light and dry in guna, heating in virya, pungent in vipaka, and, if possible, with a prabhava aimed at the channel involved. Trikatu, the three pungents, fits exactly. Facing a Pitta condition, the choice reverses: sweet, bitter, or astringent, cooling, sweet in vipaka. The same reasoning tells why a substance is not given: a cooling, heavy, sweet rasayana such as Shatavari, Asparagus racemosus, worsens Kapha and ama, undigested residue, if given while digestion is weak. Herbalism shares the same materia medica and much of the same energetic reasoning under the names hot, cold, moist, and dry.