The most intelligent class of men are called the brāhmins, and the next intelligent class of men, namely the politicians, or one who wants to take part in administration, they are called kṣatriyas. And the next intelligent class of men, those who are busy in production—because we want food—so the productive class of men is called vaiśya, mercantile.
And the laborer class of men, namely one who is neither intelligent nor administrator nor trader, but wants to live at the shelter of somebody, master, they are called śūdras. The brāhmins, the kṣatriyas and the vaiśyas, they would never accept subordination, service, under anyone. But the śūdras, as it is stated in the Bhagavad-gītā: paricaryātmakaṁ kāryaṁ śūdra-karma svabhāva-jam (BG 18.44). By nature, a śūdra wants to take shelter of somebody else and live.
So at the present moment, the Kali-yuga, the śāstra says, almost everyone is a śūdra because nobody can live independently. Kalau śūdra-sambhavaḥ. Kalau means in this age, this iron age, almost everyone is a śūdra. It is very difficult to find out really brāhmin, kṣatriya. Vaiśyas maybe there are, but mostly śūdras. Because the other three orders of life are missing, therefore at the present moment there is no peace. In every step there is fight, there is quarrel. Kali means disagreement and quarrel.
So this age is called Kali-yuga. This Kali-yuga has begun since the Battle of Kurukṣetra. You have heard in Mahābhārata or in the Bhagavad-gītā. Bhagavad-gītā is within Mahābhārata. The Battle of Kurukṣetra between the two section of cousin-brothers, the Kauravas and the Pāṇḍavas, took place under the guidance of Lord Śrī Kṛṣṇa five thousand years ago. And in that battlefield, Kṛṣṇa instructed Arjuna, His friend, the great book of knowledge, Śrīmad Bhagavad-gītā. So human life must be divided into four orders, varṇāśrama-dharma.