Ayurveda explains in detail about three basic desires of human beings, four means of knowledge, three types of human strengths, three methods of usage of sense organs, three types of diseases, three channels of diseases, three types of physicians and treatment. All these are explained in 11th chapter of Sutrasthana of Charaka Samhita, called TrisEshaniya Adhyaya.
The three basic pursuits
A person of normal mental faculty, intelligence, strength and energy, desires of his well-being pertaining to this world and the world beyond has to seek three basic desires, viz.,
Praneshana – desire to live,
Dhaneshana – desire to earn and
Paralokeshana – desire to have superior position after-death. 
Desire for longevity
Out of all these desires, one should, give priority to the desire for longevity. Why? Because, end of life means end of everything. A good life can be achieved by
Swasthasya Swastha anuvrutti – observance of healthy rules by healthy person
Aturasya vikara prashamana – treatment of the diseased.
Both these have already been described and will be described further, in detail. 
Dhaneshana – Desire for wealth
Thereafter comes the second desire for wealth. One must have a desire for wealth because there is nothing so miserable as a long life without wealth. So one must try to tap in various sources of wealth. These sources of wealth are – farming, cattle breeding, trade, government service, etc. One can adopt any other means of livelihood that is not disapproved by good men. Doing so, one lives a long life, without suffering any loss of prestige. 
Paralokeshana – Desire for good position in after-life
Doubts regarding rebirth.
Then comes the third desire to attain the happiness in after-life. This desire arises some doubts. There is doubt, whether one will have a life (rebirth) at all after death. But why to doubt? There are people who believe only in things which are perceptible and as such do not believe in rebirth because of its imperceptibility. On the other hand people believing in rebirth simply rely upon the evidence of scriptures. There are contradictory opinions with different theories like
Parent theory, nature theory, impersonal soul, free will theory etc. These theories try to explain cause of birth. So the question remains whether there is rebirth after death. 
Evidence against perception theory
A wise man should however give up these unreasonable theories and doubts. Why? Because,
Prathyaksham hi alpam – scope of perception is limited.
Analpam Apratyaksham – those that cannot be perceived by sense organs are huge.
Those un-observable things can only be perceived by
Aagama – ancient scriptures
Anumana – by inference
Yukti – by special inference, intelligence and reasoning.
Even the method of perception of knowledge by sense organs itself cannot be perceived by sense organs. 
Moreover, it is not correct to say that things which can be directly perceive only exist, and others do not. There are things, which though existent, cannot be directly perceived due to
Ati sannikarshat – a thing that is too close to eyes cannot be perceived
Ati viprakarshaat – a thing that is too far, cannot be perceived
Avaranaat – if a thing is covered by another object, then it cannot be perceived
Karana daurbalyaat – if sense organs are weak, then nothing can be perceived,
Mano Anavasthaanaat- if mind is not in place, even if a person is seeing an object, he cannot see it,
Samaana Abhiharat – confusion with other similar objects,
Abhibhaavat – over shadowing and
Ati saukshmaat – very minute object. 
Evidences against parent theory of birth
Even the critical view of scriptures cannot stand against the theory of transmigration (presence of after-life) as these are not based on proper reasoning. If the soul of mother or father enters in her/his progeny, it may be whole or a part of it. If it is transferred wholly, then certainly, the father or mother should die instantaneously. On the other hand, transformation of the subtle self in part is not possible. [9-10]
On the same principle neither the mind nor the intellect of parents can be regarded as the sole causative factor for progeny. If this theory is accepted, then the fourfold classification of species will not be possible. 
Evidence against natural theory of birth
Five basic elements and Atma possess specific qualities. (Thus, the five elements are incapable of imbibing consciousness of their own even if combined together. Consciousness is the distinctive feature of Atma alone.) Their combination and separation are conditioned by the past action of Atma. 
Views regarding soul in the creation of universe
Atma, the abode of consciousness is without any beginning. So it cannot be created. Universe is not the creator of Atma. It is the place where Atma exhibits itself. 
Views against the theory of accidental creation of universe
Nihilism constitutes the worst of the sinful. For a Nihilist, everything happens accidentally. So for him there is no existence of the soul and he does not believe in examination or in a thing to be examined; for him there is no efficient or material cause of a thing, and in his view there is no existence of Gods, sages and Siddhas (those who have attained salvation/perfection) and there is no theory of cause and effect or action and results, with a nihilist. [14-15]
So a wise person should get rid of the contempt way of thinking of a nihilist and should see things properly with the lamp of wisdom offered by good men. 
Pramanas – The four means for getting correct knowledge
All things of the universe can be divided into two.
Sat – true / existent
Asat – untrue / non existent
These can be examined by means of –
Aptopadesha – scriptural testimony (words of enlightened, realised souls),
Pratyaksha – direct perception using sense organs
Anumana – inference, guessing with reasoning
Yukti – reasoning with intelligence. 
Sages, enlightened souls –
Those enlightened and pure persons who are
Rajastamobhyam nirmutkaha – completely free from Rajas and Tamas
Tapo Jnana Balena – by the strength of penance (deep meditation) and knowledge, who always possess uninterrupted knowledge of past, present and future are known as authorities (Apta).
They are also known as desciplined, (Shishta) and enlightened persons (vibuddha).
Their words are true beyond doubt. How could such persons, being free from Rajas and Tamas, lie? [18-19]
The knowledge / intelligence acquired by the unison of Atma (soul), Indriya (sense organs), Manas (mind) and artha (object of sense organs) is called as Pratyaksha (perception or direct observation, with the help of sense organs). 
Related: How knowledge is gained through sense organs
Inference is dependent on perception. It is of three types. Related to the present, past and future.
Inference of present thing – fire is inferred from the smoke
Inference of past thing – sexual intercourse is inferred by pregnancy.
Inference of future – inference of future-tree by looking into seed, on the basis of the frequent observation about the production of fruits from seeds through direct perception. [21-22]
Example of Yukti:
Yukti is the fourth means of knowledge.
Example: Growth of crops from the combination of irrigation, land, seed and seasons;
Formation of embryo from the combination of six Dhatus (five Mahabhutas and Atma):
Production of fire from the combination of the lower-fire-drill, upper-fire-drill and the act of drilling;
Cure of diseases by fourfold efficient therapeutic measures. [23-24]
Yukti – Definition of intellect based reasoning:
The intellect which perceives things as outcome of combination of multiple causative factors, valid for the past, present and future, is known as Yukti (reasoning). This helps in the fulfilment of the three objects of human life, i.e., righteousness (Dharma), wealth (Artha) and desire (Kama). 
This is how all things-existent or non-existence can be examined and not otherwise. Such an examination establishes the theory of rebirth. 
Scriptural testimony is based on the Vedas or other scriptural material in agreement with the Vedas which is approved by the experts, gentlemen and initiated with a view to bringing about happiness to the mankind. It is derived from the words of authoritative people.
It has been stated that donation, penance, sacred rituals, truthfulness, non-violence and Brahmacharya are supposed to provide heaven and to help in liberation. (This establishes the theory of continuation of soul after death and thus of rebirth). 
Ancient sages, devoid of all human weaknesses, have clearly stated in the religious scriptures that those who could not conquer their mental defects are not eligible for salvation. (they are liable to be reborn). 
The theory of rebirth has been approved after careful observation by ancient sages endowed with divine faculty. These sages were devoted to the path of virtue; they were devoid of fear, attachment, hatred, greed, confusion and vanity; they were in tune with the Almighty; they were reliable par excellence and were conversant with the principles of ‘Karma’ or action; their mental and intellectual faculties were never tarnished. So one should not doubt this theory. 
Proving rebirth by means of Pratyaksha (direct perception)
Birth of children dissimilar to their parents; parentage and other factors being the same, difference in complexion, voice, shape, mind, intellect and fate;
Birth in high and low family; slavery and sovereignty; happy and miserable life;
Difference in the span of life; enjoyment of results without the corresponding action in this life;
Actions of a newborn like crying, sucking breast, laughing, fear etc., even without training,
Appearance of marks in the body indicating good or bad fortunes;
Action being the same, difference in its results, intuitive interest in certain types of work in some persons and not in others; Jatismarana – Preservation of memory in some persons of previous life;
People having similar external appearance but being favoured / hated.
All these confirm the theory of re-birth. 
Theory of rebirth confirmed by Anumana (Inference)
The action performed in the previous life which is unavoidable, eternal and having continuity is known as fate. Its results are enjoyable in this life. Action performed in this life will bring about its results in its future life. The seed is from the fruit and the fruit from the seed. 
Theory of rebirth confirmed by Yukti (intellect based reasoning)
The embryo is formed out of the combination of the six Dhatus (5 basic elements plus Atman). Actions are manifested by the combination of the agent (Katru) and the instrument (Karana); the results come out of the action performed. There can be no germination without a seed. The result always corresponds to action. A seed cannot bring out heterogeneous products. 
Conclusion regarding the theory of rebirth
So all the four means of knowledge establish the theory of rebirth. One should, therefore, have faith in religious scriptures; one should attend to the services of the teacher, studies, performance of religious acts, marriage, production of children, maintenance of servants, respect to guests, donations, abstinence from selfish motives, penance, avoidance of backbiting, good physical, verbal and mental acts, introspection with regard to body, sense faculties, mind objects (of senses), intellect and self, and meditation, and other similar acts recommended by virtuous persons which are conducive for doing good in the life, and after death are known as the accepted means of livelihood.
A person, attending to these acts, earns fame in this world and attains heaven after death. Thus the third basic desire relating to the life-beyond is explained. 
1. Trayopasthambha – three factors supporting life,
2. Trividha Bala – Three types of strength,
3. Tri Ayatana – three types of causes,
4. Tri Roga – three types of diseases,
5. Trayo Rogamarga – three channels of disease manifestation
6. Trividha Bhishaja – three types of physicians and
7. Trividha Aushadha – three types of medicine / treatment. 
1. Trayopasthambha – three factors supporting life
The three supports of life are
Ahara – food
Nidra – sleep and
Brahmacharya – Celibacy
Healthy habits pertaining to food, sleep and ceilbacy leads to
good complexion and growth,
full health till the full span of life. 
2. Trividha Bala – Three types of strength
Sahaja – constitutional, based on one’s body type. It exists in the mind and body from the very birth.
Kalaja – due to seasons (in winter, one has more strength) / Age (like youth having more energy)
Yuktikruta – acquired (say, by having Panchakarma, rejuvenation treatment), by combination of diet and regimen. 
3. Tri Ayatana – three types of causes and types of sensory stress
These are the three types of causes (of diseases) –
Atiyoga – excessive utilisation,
Ayoga – non-utilisation and
Mithya Yoga – wrong utilisation of
Kala – time, Artha – objects of sense organs and Karma – acts /deeds.
For example, excessive gazing at very bright substance would constitute excessive utilisation of the visual objects.
Not looking at anything at all would amount to its non-utilisation.
Seeing things that are too close or too far away, looking at awful, terrifying, surprising, contemtpuous, frightful derforme and alarming this is wrong utilisation of sense organ. Excessive utilisation of auditory objects would be to hear uproarious noise coming out of thunder and kettle drum, loud cries, etc.; its non-utilisation would be not to hear anything at all; hearing of harsh words, news about the death of friends, assaulting, insulting and terrifying sounds constitute the wrong utilisation.
Smell of exceedingly sharp, acute and intoxicating odours constitute Atiyoga – excessive utilisation of olfactory sense faculties,
not to smell at all is its Ayoga – non-utilisation;
Smell of exceedingly putrid, unpleasant, dirty and cadaverous odour and poisonous gas is Mithyayoga (wrong utilisation).
Similarly excessive intake of various substances having various tastes would amount to Atiyoga of tongue,
Not tasting anything – is Ayoga and
tasting obnoxious things Mithya yoga.
Excessive use of exceedingly cold and hot bath, massage and unction etc., amounts to Atiyoga of the tactile sense faculty;
Not to use it at all is Ayoga;
Use of bath, massage and unction and other hot and cold substances without observing the prescribed order, touch of uneven place, dirty objects, microbes and injurious touch constitute Mithya Yoga. 
Mode of operation of sensory stress:
The touch sense organ pervades all other organs. It is spread allover the body. It is continuously associated with the mind. The mind again pervades the sense of touch. So the unfavourable reaction of all the senses caused by the all pervasive sense of touch is known as the unwholesome conjunction (sensory stress). The objects of sense faculties which are of five types are further sub-divided into three each (Atiyogga – excessive utilisation, Ayoga – non-utilisation, and Mithyayoga – wrong utilisation). The favourable reaction of the senses is regarded as the wholesome conjunction of the senses with their objects. 
Types of unwholesome action:
Action includes speech, thoughts and deeds.
The examples of wrong utilisation of the body are
Vegadhaarana – suppression of natural urges,
Vega Udeerana – forceful initiation of natural urges,
unbalanced slipping, falling and posture; excessive itching of the body, bodily assault, excessive massage, excessive holding of breath and exposing oneself to excessive torture.
Examples of wrong utilisation with regard to speech are back-biting, lying, useless quarrels, unpleasant talk, irrelevant unfavourable talks and harsh expressions.
Those relating to mind are fear, anxiety, anger, greed, confusion, vanity, envy and misconception. 
In brief, any action relating to speech, mind and body which is not included either in the categories of excessive utilisation or non-utilisation, and which is harmful for the health in the present life and which is against the religious prescriptions comes under the category of wrong utilisation of speech, mind and body. 
So three-fold actions (i.e. relating to speech, mind and body) further divided into three categories (in the form of non-utilisation excessive utilisation and wrong utilisation) constitute intellectual misuse. 
A year is the unit of time which is further sub-divided into winter (Hemanta), summer (Grishma) and rains (Varsha) characterised by cold, heat and rainfall respectively.
If a particular season manifests itself excessively, this should be regarded as Atiyoga; example, extremely heavy rains during rainy season.
if the season manifests itself in lesser measure, it would be its Ayoga. Eg: less rains in rainy season
If characteristics of a season are contrary to the normal ones, this would be wrong utilisation (for example rainfall in winter, cold in the rainy season, etc.). 
Three causes of
Asatma Indriyartha Samyoga – unwholesome conjunction of the sense organs with their objects,
Prajnaparadha – doing wrong deeds even with the knowledge
Parinama – transformation / conversion – these are the threefold causes of diseases.
Proper utilisation of the objects, action and time is beneficial to the maintenance of normal health. 
Proper maintenance or otherwise of various items of creation depends on proper utilisation, non-utilisation, excessive utilisation and wrong utilisation of certain conditions because both proper maintenance as well as abnormality depend on the conjunction (of wholesome or unwholesome nature). 
4. Trividha Roga – Three types of diseases
There are three types of diseases-
Nija – endogenous, due to imbalance of the internal factors of the body, caused by Dosha imbalance
Agantuja – exogenous, due to external causes like injury, fire, poison etc
Manasa – mental disorders / psychic disorders 
Principles of treatment of mental diseases:
So a wise person (even if) suffering from the mental diseases should repeatedly analyse what is useful and what is harmful to health.
He should strive for discarding the harmful or unwholesome regimens and adopt the wholesome ones in regard to righteousness (Dharma), wealth (Artha) and desire (Kama). No happiness or unhappiness can occur in this world without these three elements. So one should try to serve persons well versed in the nature and cure of psychic diseases. One should also try to acquire knowledge of the self (Atma), the place, family, time, strength and the capacity. 
Thus it is said:
The following are be attended for the treatment of psychic diseases:
(i) To attend the course of conduct relating to virtue, wealth and desire;
(ii) To render service to the persons well versed in the nature and cure of psychic diseases;
(iii) To obtain all round knowledge about the self, etc. 
5. Trividha Rogamarga – Three paths of diseases in body
The three course of the disease are
Shakha (peripheral system) – diseases afflicting the 7 dhatus (lymph, blood, muscles, fat, bone, marrow and reproductive system)
They are also called as Bahya roga marga, (peripheral system)
Diseases afflicting this path –
Ganda (goiter), Pidaka (pimple), Alaji (boil), Apachi (scrofula), Charmakeela (wart), Adhimamsa (granuloma), Mashaka (moles), Kushta (skin diseases) and Vyanga (freckles), Visarpa (herpes), Shvayathu (oedema), Gulma (abdominal tumour), Arshas (piles) and Vidradhi (abscess)
Marmasthisandhi (vital organs and joints of bones), – diseases involving vital organs (marmas) like Basti – bladder, heart, bone joints and related tendons and ligaments etc
This path is called as Madhyama roga marga.
Usual diseases occurring in this path are –
Pakshavadha (hemiplegia), Pakshagraha (stiffness), Apatanaka (convulsion), Ardita (facial paralysis), Shosha (consumption), Rajayakshma (tuberculosis), Asthi-sandhisula (pain in the bone joints), Gudobhramsha (prolapse rectum) and the diseases of the head, heart and bladder.
Kostha (gastro intestinal system) – diseases caused in digestive tract – stomach, intestines etc., from mouth to anus. 
This is known as internal path (Abhyantara rogamarga)
Ailments like Jvara (fever), Atisara (diarrhea), Chardi (vomiting), Alasaka (intestinal torper), Visuchika (cholera, Kasa (cough), Shvasa (dyspnoea), Hikka (hiccough), Anaha (constipation), Udara (diseases of the abdomen), and Pliha (splenic disorders) and the internal variety of Visarpa (skin diseases characterized by an acute spread), Shvayathu (oedema), Gulma (abdominal tumour), Arshas (piles) and Vidradhi (internal abscess) belong to the central system. 
6. Trividha Bhishaja – Three types of Physicians
Chadmachara – quacks, pseudo physicians. They pose themselves as doctors by exhibiting a few medical books, medicine box etc. They are ignorant about medical science.
Siddhisadhita – feigned physicians, who practice just with the help of experience. They claim to have wealth, fame and knowledge. But they do not have any of these.
VaidyaGuna Yukta – Good ones, endowed with all the qualities.
Those who are accomplished in the administration of therapies, insight and knowledge of therapeutics are endowed with infallible success and can bring out happiness to the patient are saviours of life. [50-53]
Three types of treatments:
Daiva Vyapashraya – Spiritual treatment –
Mantra (holy recitations), Mani (gem therapy), Mangala Bali, Homa – good deeds, Fire rituals, Niyama – Self restrictions like fasting, reconciliation, holy chanting,
Yukti Vyapashraya – therapy based on reasoning and planning. Administration of proper diet and medicinal drugs comes under the second category.
Satva Avajaya – , Withdrawal of mind from harmful objects constitutes psychic therapy. 
7. Trividha Aushadha – Three types of therapies
When there is Dosha imbalance (=disease) in the body, generally three types of therapies are required. viz.,
Antah Parimarjana – internal-cleansing, useful in diseases caused by improper diet.
Bahi Parimarjana – external-cleansing. Like Abhyanga (massage),Sveda (fomentation), Pradeha (unction), Parisheka (dripping), Mardana (kneading) etc.
Shastra Pranidhana – surgical therapy. Chedana (excision), Bhedana (incision), Vyadhana (puncturing), Darana (rupturing), Lekhana (scraping), Utpatana (uprooting), Pracchanna (multiple puncturing), Seevana (rupturing), Eshana (probing), application of (Kshara) alkalies and Jalauka (leeches). 
Importance of management of diseases
In the event of a disease, a wise person regains his health by administering external and internal cleansing therapies and also by surgical procedures. However, as an incompetent king neglects his enemy, so also an ignorant person does not realise the need to take care of the disease in its primary stage because of his negligence. This disease, in its early stage appears to be insignificant, but it grows and thereafter gaining a strong hold on the body. It takes away the strength and life of the fool. The fool is never conscious of any defect unless he is seriously afflicted. After he is actually afflicted seriously, he thinks about disease eradication. Then he calls his children, wives and requests them to call in a physician and says, “I am prepared to pay him my entire earnings.” But then, who can save such a weak, emaciated, wretched and moribund person afflicted with diseases and with his sense organs losing all the strength. Having failed to find a Saviour of his health, the fool is deprived of his life (in spite of his efforts to preserve it) like an inguana with her tail bound by a rope being dragged by a strong person. So a wise person, desirous of his well-being, should take recourse to the appropriate therapies before the occurrence of the diseases or even while the diseases are in their primary stage of manifestation. [56-63]
To sum up-
Basic desires, supporters, strength, causes (of diseases), diseases themselves, paths, physicians and therapies – all these eight factors – each classified into three groups have been described in this chapter by the sage Krsnatreya who is wise and free from worldly attachments. Everything (virtue, wealth and desire) is based on these eight factors. [64-65]