Owing to the importance of the Trisutra (Triad Principle), the proper understanding of the same becomes more imminent. It was this principle that was given by Lord Indra to Sage Bharadwaja (Charaka Samhita Sutrasthana 1st Chapter). This shows that it is the methodology for understanding Ayurveda that was given by Lord Indra that was adopted by Saint Bharadwaja and others further. This is also applied in every content of Ayurveda making it more easy to understand and also for application. Now let us understand the principle in detail –

HETU (CAUSE)

The term HETU refers to cause, purpose, argument, reason for inference, primary cause, logic, etc in the Sanskrit dictionary.  With respect to Ayurveda it refers to all these meanings depending on the context. For example, if it is a description of the disease then it refers to the causes of disease, while if it is the description of the concept, then it refers to the query or logic based on which the concept is developed and so on. Based on the definition of Trisutra, it is evident that all the three components of the Trisutra are broadly considered to be of two categories namely 

1. Causes that help to maintain Swastha or Health.
2. Causes that help to cause Roga or Disease.

But when the analysis of the context in which the reference is placed is done, it shows that there is another one category of HETU namely – 

1. The logic due to which the concepts or principles of Ayurveda has been developed.

The three together form the Principle or sutra of Hetu in Ayurveda. It is important that all the causes are properly known and understood for a thorough understanding of Ayurveda and its applications. The three categories of Hetu are important to have the complete theoretical and practical knowledge. If any of these three are ignored then the knowledge of Ayurveda cannot be completed.

The logic are the factors that shape the concept to have a meaningful and proper genesis. They are understood by the help of the Pramanas (Tools of Knowledge). The logic thus forms the basis of each and every science. The logic or HETU when properly understood will lead to the appreciation of the principle making it more acceptable. It is only when one understands and accepts the principle or concepts of a science the application is possible so as to obtain the desired benefits. This is the one that stands for the Shastrartha (Proficiency in theoretical aspects of the science) property of the physician. Here the HETU is to be taken as three types namely Samavaya, Asamavaya and Nimitta. These are nothing but the 3 varieties of Karana or cause mentioned in Karya-Karana principle.

Other than this HETU, the other two varieties of HETU as mentioned above. These are understood by the application of principles and concepts of Ayurveda. The understanding of these causes will be useful in achieving the main aim of Ayurveda that is maintenance of health and Curing of disease. This is the one that stands for the Drushtakarma (Proficiency in Practical aspects of the science by applying the concepts and principles in a proper and logically method). Accordingly, the causes are Trividha Nidana (Kala or time, Indriyartha or Perception of sense organs and Karma or various activities or functionalities) when performed in an improper manner will lead to disease while done in an optimum way will maintain the health.

LINGA (CHARACTERISTIC FEATURES)

The LINGA principle is the one that explains characteristic features of a given concept, condition or disease according to the context. The word LINGA refers to symptom, sign, mark, feature, etc with respect to the present context. Thus, the principle can be reclassified into two major groups namely – 

1. The LINGA or characteristic features of the concept that has been formulated. 
2. The LINGA or the signs and symptoms of the healthy and unhealthy conditions. 

The first variety is actually the signs or features which is exhibited by the concept like the actions, beneficial or harmful effects or even clarity of certain activities. These features are those that act as an identity of the concept and will act as the parameters of the assessment of the concept too. Thus, the Hetu or the cause helps in understanding the reason for the emergence of the event or situation or the concept while understanding of Linga or Characteristic features give the idea of the concept and the methods to evaluate it. 

The second variety is those set of signs and symptoms that are related to the different conditions like health and disease. This may include the signs that are general or specific with respect to a given condition of the disease or healthy state. It may also represent the condition that is considered as a sign of impending disease or different stages of recovery too. The basis of the understanding is through the concepts and principles of the science too.  It proper understanding and interpretation will lead to the next stage or principle namely AUSHADHA or solutions or remedies.

AUSHADHA (SOLUTIONS OR REMEDIES)

The final effect of analysis is the knowledge of proper solution or remedy. This is possible only when the other two aspects or principles are clear. Even this is naturally to be of two types namely – 

1. Solutions to the queries or the cause that have resulted in the origin of principles or concepts. 
2. Remedies for the conditions that result in maintenance of health or cure of disease.

In the first instance the solution to a given problem which is conceptual in nature is the effect of the generation of concept. It is reproducible in various similar situations which makes the concept more acceptable and true. It itself is a solution to many queries with respect to disease and health but its effect on the query is the logical remedy or solution that the concept gets as a result of the analysis and understanding. 

In the second instance the Aushadha refers to medications, procedures and treatment modalities that helps in the maintenance of health or even relief from a disease condition. Accordingly, the one that results in solution or remedy of disease is of two types namely Shodhana & Shamana. The one that are useful in the management of health are known as Swasthahita. 

Thus the three principles together give a complete idea of the Ayurvedic science. Their in-depth analysis and understanding is the much required aspect for the understanding, practice and have the optimum benefit with respect to Ayurveda.