So this birth you can get. This human form of birth you can get. That is the special significance. Father, mother, you can get any birth. It doesn't matter. But you cannot get Kṛṣṇa and guru. That you can get in this birth. Therefore twice-born. You must get another birth, through the initiation of the spiritual master, dīkṣā. Dīkṣā means initiation. Di means divya jñānam, and kṣa means kṣapayati. From the day of initiation, you simply get spiritual knowledge, transcendental knowledge. That is the special significance of human form of body. Therefore in the Vedic civilization, the classification is made: first-class, second-class, third-class, fourth-class men.
There are three guṇas. The material world is conducted by three guṇas, means the quality of goodness, the quality of passion, the quality of ignorance, and mixed. So first, second, third, and the mixed is called fourth. That is called varṇāśrama. Cātur-varṇyam. Guṇa-karma-vibhāgaśaḥ (BG 4.13): according to quality and karma. So the first-, second-, and third-class division, they are all dvijas, twice-born. Brāhmaṇa, kṣatriya and vaiśya. Dvija means saṁskāra, reformation, to mold the character. That is called saṁskāra.
Just like you can train even a tiger, even an animal, to dance according to your desire. That is practical. So you cannot train a human being to become a brāhmaṇa? With such intelligence? But there is no such training. They are simply training all cats and dogs. But the Vedic civilization takes this opportunity of human being, to make him perfect in his life. What is that perfection? That perfection is to understand God and become His devotee. That is perfection. Not that to become very stout and strong, as strong as the tiger or as cunning as the jackal. Not that. That is not training.