A plain language summary of the paper is provided at the end for the benefit of the lay reader. -- Editors
तन्न सम्यक् यतस्तत्तु
हेत्वाभास इति स्थिति:।।
First of all, I have concluded, without perhaps a need to revisit, that the tridosha theory, as contained in the classical texts of Ayurveda, is a hetvaabhasa (an instance of supposing false causal relationships). Biological functions, both in health and illness, have been grouped into three categories based upon their observable concomitance. Definitive bio-chemical entities called doshas have then been pre-supposed to exist underlying this categorical concomitance of biological functions. The problem comes here, for what ought to have been a supposition of inference, has merely been a supposition of speculation. Consequently, the theory as it is found in the classical texts, if taken unmodified, is unscientific.
It would, however, be hasty if one rubbished it away altogether. The dosha theory, clinical experience insists, is highly useful in carrying out a holistic assessment and a personalised management of the patient. In view of this utility, the doshas may tentatively be taken as theoretical constructs and the theory itself, as a rough and ready one. Needless to say, such a theory cannot function with mathematical precision and is consequently unsuitable for extrapolation in high-risk clinical situations.
While it is true that the bio-chemical entities (doshas) underlying the concomitance of biological functions are indeed speculative, the concomitance itself does not seem to be. Common experience and clinical observations suggest a roughly consistent association of physiological features (in prakriti) and of pathological manifestations (in vikriti). What accounts for this association is a question that needs investigation. The investigation itself needs to be bi-phasic.
The first phase should ascertain whether the concomitance is real or arbitrary. This is important because an employer of a theory is often unwittingly susceptible to confirmation bias. As Sherlock Holmes would say, "Insensibly one begins to twist facts to suit theories, instead of theories to suit facts".
Once the concomitance is established, the next phase would be to investigate the Physics and Chemistry underlying it. This investigation would yield path-breaking results with unspeakably big ramifications for the way Medicine is being learned and practised. But, that is at quite some distance ahead.
If the concomitance is seen to be a result of biased observation of the sort I have mentioned earlier, the theory will have to be discarded in its entirety. It would still be path-breaking for Ayurveda, because the dogmatic baggage of three millennia, would finally be jettisoned away. (This, in my view, is an unlikely scenario.)
Ayurveda has employed the tridosha theory for two distinct purposes. Although this distinction has no basis in the classical texts, it becomes a logical imperative to suppose it as the corollary of my findings explained above.
Rationalisation (logical systematisation) of verified clinical experience is the first and central purpose for which the theory has been employed. Doshas, in their role here, may simply be regarded as theoretical constructs meant to classify clinical features and facilitate right therapeutic choices thereafter. A verse explaining the contra-indications of physical exercise, for instance, reads वातपित्तामयी बालो वृद्धोऽजीर्णी च तं त्यजेत्। The word वातपित्तामयी in this line refers to an observable category of clinical features where exercise does harm. Similarly, the idea of dosha-prakriti, for another instance, relates to clinically observable and evidently distinct categories of phenotypical features.
The second purpose for which the dosha theory has been employed is to explain (away!) the physiological and pathological processes. A verse like आमाशयाश्रयं पित्तं रंजकं रसरंजनात्। is meant to conjecture a life-process, blood formation in this case. Such a role of doshas is inconceivable without considering them in their originally supposed status as biochemical entities.
Now, the direct corollary of my finding is that the tridosha theory retains relevance only in its former aspect. It retains value primarily for its role in logically systematising verified medical experience. The theory's latter role in conjecturing physiological and pathological processes however ought to be obsolete.
The extrication of the relevant aspect from the obsolete one, though often as straight-forward as in the instances mentioned, is not always so. Where it is not, the extrication might be accomplished keeping in mind the imperative stated above. It would effectively mean a separation of sound medical observations from a formidable web of physio-pathological conjectures.
Plain Language Summary
The classical texts of ayurveda present the tridosha theory as a complete law of patho-physiology. This study demonstrates the prudence of seeing it as merely a rough-and-ready model that the ayurvedic sages devised to systematise their medical experience. Such a model is simply the result of reasoning intuitively - of using rules of thumb to simplify problems for the sake of efficiency. It relies on common-sensical shortcuts that have arisen as handy ways to solve complex cognitive problems rapidly, but at a cost of inaccuracies and misfires. Needless to add, such a model cannot account for observations that are counter-intuitive. The methods of science and statistics grew up precisely to check these obvious limitations of intuitive reasoning. It is time to employ these methods to verify and if possible, to refine this apparently useful theory.
Howsoever grandiose the classical conception of doshas might be, they are relevant to us, until further research, only as rough theoretical constructs meant to classify clinical features and facilitate right therapeutic choices thereafter. Whether the basis for this classification of clinical features is pathogenetic similarity, as the classics hold, or mere therapeutic similarity naively over-extrapolated to imply pathogenetic similarity, will have to be researched. The possibility of there not being a tenable basis at all appears somewhat remote.
तस्मात् सतामत्र न दूषितानि
मतानि तान्येव तु शोधितानि ।
 Also, the likelihood of evident therapeutic similarity being over-extrapolated in many instances to conjecture pathogenetic similarity cannot be ruled out.
 The Scientific Reports paper entitled, “Genome-wide analysis correlates Ayurveda Prakriti” is an example of a work in this direction.