Satya Yuga, also called Krita Yuga, is the first of the four ages that make up a mahayuga or great age in Hindu cosmology, and it is given a duration of 1,728,000 years, four units of 432,000. The texts describe it as the age in which dharma, right order, stands on all four legs; there is no disease, no falsehood, no need of scripture or of law, no division into castes in some accounts, and human life is enormously long. The scheme is set out in the Manusmriti, in the Mahabharata and in the Puranas, with the Vishnu Purana giving one of the fullest statements. The proportions are the key structural feature: the four ages stand in the ratio 4:3:2:1, so that each is shorter than the last and dharma loses a leg with each, and the whole is an account of decline rather than of progress.
The Bhagavata Purana gives the bull of dharma four specific legs, tapas or austerity, shaucha or purity, daya or compassion and satya or truth, and its account of the decline across the ages names which legs fail first: by the Dvapara Yuga, tapas and shaucha are already diminished, and by the Kali Yuga daya has failed as well, leaving only a bare remnant of satya standing, which is why the present age is described as clinging to truth even as everything else the bull once stood on has been lost. In the Puranic scheme of the ten avataras of Vishnu, the first four, Matsya the fish, Kurma the tortoise, Varaha the boar and Narasimha the man lion, are traditionally placed within Satya Yuga, though the texts are not perfectly uniform on where one age's avatara ends and the next begins.