It is said that Dharma has four legs (pillars) and it is depicted in the form of vrushabham (nandi). What are those four legs? As far as I know, cow's four legs are also seen as four legs of Dharma.
Dharma has 4 legs in Satya yuga, 3 in Treta, 2 in Dwapara and just 1 leg in Kali yuga. How to understand this?
Does it merely mean the percentage of righteousness in an individual, righteous people in a society or a theoretical combination of some practices?
No, the four legs of the bull of Dharma do not represent percentages of righteousness in society (although it is true that the number of righteous people decline as the Yugas progress). Rather, the legs represent different virtuous qualities any individual can have. Here is what Arjuna's grandson Parikshit says in the Srimad Bhagavatam, upon seeing the bull of Dharma standing on one leg at the start of the Kali Yuga:
In the age of Satya your four legs were established by the four principles of austerity, cleanliness, mercy and truthfulness. But it appears that three of your legs are broken due to rampant irreligion in the form of pride, lust for women, and intoxication. You are now standing on one leg only, which is your truthfulness, and you are somehow or other hobbling along. But ... [Kali], flourishing by deceit, is also trying to destroy that leg.
So to sum up, in the Satya Yuga, the bull of Dharma starts out with four legs: Tapas or austerity, Shaucha or cleanliness, Daya or mercy, and Satya or truth. By the start of the Treta Yuga, austerity is gone, and by the start of the Dwapara Yuga cleanliness is gone. Finally, by the time the Kali Yuga comes, only one leg is left standing, namely truth, and even truth is steadily eroded as the Kali Yuga progresses.
By the way, on a positive note, Parikshit was able to re-establish the legs of Dharma in the Kali Yuga, by banishing the Kali Yuga into a limited number of places:
Mahārāja Parīkṣit, thus being petitioned by the personality of Kali, gave him permission to reside in places where gambling, drinking, prostitution and animal slaughter were performed. The personality of Kali asked for something more, and because of his begging, the King gave him permission to live where there is gold because wherever there is gold there is also falsity, intoxication, lust, envy and enmity. Thus the personality of Kali, by the directions of Mahārāja Parīkṣit, the son of Uttarā, was allowed to live in those five places. Therefore, whoever desires progressive well-being, especially kings, religionists, public leaders, brāhmaṇas and sannyāsīs, should never come in contact with the four above-mentioned irreligious principles. Thereafter the King reestablished the lost legs of the personality of religion [the bull], and by encouraging activities he sufficiently improved the condition of the earth.
So as long as you avoid those five things, the negative effects Kali Yuga won't be present. And as I discuss in this answer, the Kali Yuga has its own benefits, like the ability to achieve great results simply by chanting the name of Vishnu.
The Varaha Purana, Chapter 32, The Origin of Dharma (deva), explains the complete characteristics of Dharma and how he (or it) was initially lost and then restored:
CHAPTER EIGHTEEN (The Origin of Fire)
Prajāpāla said:
1-3. О great sage, how did Agni, Asvins, Gaurl, Ganapati, Nagas, Skanda, Aditya, Candra, Matrs, Durga, Diks, Kubera, Visnu, Dharma, Siva, Pitrs and Candra take birth? How did all these deities of the body attain concrete forms?
CHAPTER THIRTYTWO (The Origin of Dharma)Mahātapas said:
Now listen, О king, I shall tell you about the origin of Dharma, its greatness and the tithi associated with it.
Formerly, the eternal and pure Brahma, who is inferior only to the Supreme, thought of creation as well as the protection of the created.
Then from the right side of his body came out a person with body smeared with white unguent and wearing white garland and white earrings.
- Seeing him who was four-footed and bull-shaped, he said: "You be the foremost of my creation and protect all these beings".
- Thus ordained, he protects the world with all the four feet in Krtayuga, three feet in Tretayuga, two feet in Dvaparayuga and a single foot in Kaliyuga.
- Dharma was established with six divisions in Brahmins, three in Ksatriyas, two in Vaisyas and one in Sudras. This lord (Dharma) is prevalent everywhere in all the islands in the earth.
- His four feet are acclaimed as Guna (quality), Dravya (substance), Kriya (action) and Jati (genus).
- In the Veda he is considered as having three horns, Samhita, Pada and Krama. He is Omkara and has two heads and seven hands.
- He was bound by the three svaras, Udatta, Anudatia and Svarita[1]. Such was Dharma.
10-11. Dharma was persecuted by Soma (moon), desirous of (getting for himself) Тага, the wife of his own brother Angirasa, and somehow escaping from him, he resorted to the expansive forest.
12-13. When Dharma was gone, the groups of gods began to wander about desiorus of killing Asuras, being bereft of dharma. The Asuras too similarly wandered about in the abodes of gods.
- As a result of Soma's sin based on the woman, Devas and Asuras fought each other with numerous weapons.
- Seeing them at war with each other Narada came to his father Brahma and spoke about it.
- Brahma then mounted his swan and forbade them from fighting, and asked them about the cause of fight.
- They all said Soma to be the cause, and then he was able to see that his son Dharma had resorted to the forest because of persecution.
- Then Brahma went there quickly with Devas and Asuras and saw him there four-footed, bull-like and white like the moon.
Brahma said:
- This is my first son, the sage by name Dharma, harassed by the moon desirous of the wife of his own brother.
- О Devas and Asuras, propitiate him so that your former position may be established.
- Thus knowing his identity through Brahma, Devas and Asuras extolled him.
Devas said:
- "Bow to you, О lord of worlds, moon-like in complexion, bow to you with divine form, the guide to heaven's path.
- Bow to you of the form of Karmamarga (the path of action) and omnipresent. The earth is governed by you, in fact all the three worlds.
- The other worlds like Jana, Tapas and Satya are also governed by you. Without you there is nothing, moving or stationary.
25-26. Without you the world perishes in a moment. You are (the qualities) Sattva, Rajas and Tamas respectively manifesting in the good, the common and the bad people.
- Bow to you the bull-shaped one with four feet, four horns, three eyes, seven hands and three joints.
- О lord, without you, we all go astray. So give us the right path as we are ignorant. You are our great refuge."
- Thus praised by gods, the lord of beings in bull's form, became pleased and spoke with benignity.
- By the very look of Dharma Devas became freed from ignorance and attached to righteousness. So too Asuras.
31-32. Then Brahma said: "O Dharma, from today onwards let your tithi be Trayodasi. He who fasts on that day and worships you will attain you and become freed of sins.
- This forest where you have been wandering long will be known by the name 'Dharmaranya'.
- The world will see you four-footed, three-footed, two-footed and single-footed respectively in the yugas Krta, Treta, Dvapara and Kali, and extending over earth and heaven, protecting the whole universe".
- Saying this Brahma disappeared while Devas and Asuras were looking on. They too went to their homes fearlessly.
- He who recites to others this story of Dharma and he who propitiates his Pitrs with milky food on Trayodasi, goes to heaven and remains with gods.
[1] Udātta is the high tone, Anudātta the low tone and Svarita a combination of the two.
The Manu Smriti says the following about how Dharma diminishes with each passing Yuga and how it directly affects men or individuals:
1-81. In the Krita age Dharma is four-footed and entire, and (so is) Truth; nor does any gain accrue to men by unrighteousness.
1-82. In the other (three ages), by reason of (unjust) gains (agama), Dharma is deprived successively of one foot, and through (the prevalence of) theft, falsehood, and fraud the merit (gained by men) is diminished by one fourth (in each).
1-83. (Men are) free from disease, accomplish all their aims, and live four hundred years in the Krita age, but in the Treta and (in each of) the succeeding (ages) their life is lessened by one quarter.
1-86. In the Krita age the chief (virtue) is declared to be (the performance of) austerities, in the Treta (divine) knowledge, in the Dvapara (the performance of) sacrifices, in the Kali liberality alone.
In Uttarakhanda of Ramayana, Sage Narada explains to Rama how Dharma has slowly deteriorated during the course of various Yugas and how it has also shortened the life-span of people in general:
Hearing those words of the poorly king, Narada, in the presence of all other Rishis, replied :—
"Hear, king, why this boy hath met with untimely death. And hearing this, O descendant of Raghu, do thou settle what thou thinkest proper.
O king, in the golden age [Krita-yuga], only the Brahmans used to engage in asceticism. At that age, save the Brahmanas no other caste used to lead the life of an anchoret. And for this, the Brahmans were the highest caste, effulgent in asceticism, shorn of ignorance, above death and conversant with three ages.
Thereupon at the end of the golden age, Brahmana's understanding became loose and the Treta Yuga set in. At this age Kshatryas, gifted with the power acquired by pristine asceticism, were born. And those men, intent upon austere penances, that were born in the Treta age, were more powerful and greater devotees than those in the former age. In the golden age the Brahmanas were more powerful than the Kshatryas. But in the Treta age the Brahmanas and the Kshatryas were equally powerful. Thus in the Treta age, not beholding prominence of the Brahmanas greater than that of the Kshatryas, Manu and other religious leaders of the time composed the Sastra describing and getting in the four divisions of castes. In this wise the Treta age was prolific with virtues of the four Varnas, and many pious sacrifices and was shorn of iniquities.
But being attacked a little with iniquity, one portion of sin appeared on earth. And growing impious people lost their strength. And for houses and farms that were the property of the former age, people of the Treta Yuga were possessed by envy, the outcome of the quality of darkness. And with the fading of impiety on earth during Treta, the soiled sin of untruth appeared. And this iniquity stretching one footstep, the lease of people's lives, for sins, became limited. And the sin of untruth descending on earth, people, to avoid the extinction of life in consequence thereof, became truthful and engaged in many pious observances. During the Treta age the Brahmanas and Kshatryas engaged in austere penances and the Vaisyas and Sudras engaged in serving them. And the greatest piety of the Vaisyas and Sudras at that time consisted of serving the Brahmanas and Kshatryas and specially for the Sudras the highest religion was to serve people of all, Varnas. O foremost of kings, at the end of the Treta age, Vaisyas and Sudras being fully overpowered by the sin of untruth, the Brahmanas and the Kshatryas also grew feeble.
And the second foot of impiety being ushered into the world the Dwapara age set in. O foremost of men, during this age two legs of piety being cut off, impiety and untruth multiplied. And in Yuga named Dwapara the Vaisyas engaged in devout penances. In this wise in three ages the three Varnas gradually engaged in ascetic performances. The piety of asceticism by Yugas gradually became established in three Varnas. But, O foremost of men, in these three Yugas the Sudras were not entitled to the virtues of devout penances O foremost of men, the degraded caste—the Sudras, during thy regeme, have engaged in austere penances. And in the Kali Yuga asceticism shall be established in the Sudras. O king, even in the Dwapara, devout penances for Sudras were considered as impiety, what to speak of the Treta age. O king, one Sudra, under the influence of vicious understanding has begun devout penances within thy kingdom. And for that reason this boy hath met with death. Calamity sets in that kingdom where a vicious-minded person commits an iniquity and that vicious wight and the king forsooth speedily repair to hell.
In Vana Parva of Mahabharata, Sage Markandeya also explains to Yudhishthira, how lack of Dharma shows its signs in each and every individual in the society:
O bull of the Bharata race, in the Krita age, everything was free from deceit and guile and avarice and covetousness; and morality like a bull was among men, with all the four legs complete. In the Treta age sin took away one of these legs and morality had three legs. In the Dwapara, sin and morality are mixed half and half; and accordingly morality is said to have two legs only. In the dark age (of Kali), O thou best of the Bharata race, morality mixed with three parts of sin liveth by the side of men. Accordingly morality then is said to wait on men, with only a fourth part of itself remaining. Know, O Yudhishthira, that the period of life, the energy, intellect and the physical strength of men decrease in every Yuga! O Pandava, the Brahmanas and Kshatriyas and Vaisyas and Sudras, (in the Kali age) will practise morality and virtue deceitfully and men in general will deceive their fellows by spreading the net of virtue. And men with false reputation of learning will, by their acts, cause Truth to be contracted and concealed. And in consequence of the shortness of their lives they will not be able to acquire much knowledge. And in consequence of the littleness of their knowledge, they will have no wisdom. And for this, covetousness and avarice will overwhelm them all. And wedded to avarice and wrath and ignorance and lust men will entertain animosities towards one another, desiring to take one another's lives. And Brahmanas and Kshatriyas and Vaisyas with their virtue contracted and divested of asceticism and truth will all be reduced to an equality with the Sudras. And the lowest orders of men will rise to the position of the intermediate ones, and those in intermediate stations will, without doubt, descend to the level of the lowest ones. Even such, O Yudhishthira, will become the state of the world at the end of the Yuga.
I think the order of the padas of dharma is also important. First comes truthfulness, then comes non-violence/mercy, then comes cleanliness, and finally austerity. One theory is each of these legs are lost during each yuga as shown in this link: http://www.harekrsna.com/sun/editorials/01-12/dharma.pdf
Another way of understanding this is losing each of these four traits by 1/4th every yuga. In Sanskrit, pada also means a quarter. So in Satya yuga, 100% of all four (truthfulness, non-violence/mercy, cleanliness and austerity) is followed. In Treta yuga the compliance was 75% or less, in Dwapara it became 50% or less. So in the present Kali yuga, the compliance of Dharma is 25% or even less.
Keshav Srinivasan pointed out a beautiful fact that we can escape the effects of Kali by chanting the names of Lord Vishnu. Scriptures unambiguously say that we can completely get rid of the ill effects of Kali yuga with singing of Krishna sankirtana (the constant chanting of Krishna's name).
Here is how it's explained about four legs in the original MahAbhArata during Vana Parva:
Yudhishthira, the son of Kunti, once more asked the great Muni Markandeya about the future course of the government of the Earth. ... Markandeya said, '... O bull of the Bharata race, in the Krita age, everything was free from deceit and guile and avarice and covetousness; and morality like a bull was among men, with all the four legs complete. In the Treta age sin took away one of these legs and morality had three legs. In the Dwapara, sin and morality are mixed half and half; and accordingly morality is said to have two legs only. In the dark age (of Kali), O thou best of the Bharata race, morality mixed with three parts of sin liveth by the side of men. Accordingly morality then is said to wait on men, with only a fourth part of itself remaining. Know, O Yudhishthira, that the period of life, the energy, intellect and the physical strength of men decrease in every Yuga!
So this entirely means that, righteousness in the society decreases (due to increase in sin) as you have guessed perfectly in your Qn!
