In the Bhagavad-gita Kṛṣṇa says, catur-varnyam maya srstam guna-karma-vibhagasah: “I created four divisions of men according to their quality and work.” [Bg. 4.13] For instance, you can understand that there are engineers as well as medical practitioners in society. Do you say they belong to different castes-that one is in the engineer caste and the other is in the medical caste? No. If a man has qualified himself in medical school, you accept him as a doctor; and if another man has a degree in engineering, you accept him as an engineer. Similarly, the Bhagavad-gita defines four classes of men in society: a class of highly intelligent men, a class of administrators, a class of productive men, and ordinary workers. These divisions are natural. For example, one class of men is very intelligent. But to actually meet the qualifications of first-class men as described in the Bhagavad-gita, they need to be trained, just as an intelligent boy requires training in a college to become a qualified doctor. So in the Kṛṣṇa consciousness movement people are being trained how to control their minds, how to control their senses, how to become truthful, how to become clean internally and externally, how to become wise, how to apply their knowledge in practical life, and how to become God conscious. The movement is not introducing the caste system, in which any rascal born in a brahmana family is automatically a brahmana. He may have the habits of a fifth-class man, but he is accepted as first class because of his birth in a brahmana family. This is not accepted. A man is recognized as first class who is trained as a brahmana. It doesn’t matter whether he is Indian, European, or American; lowborn or highborn-it doesn’t matter. Any intelligent man can be trained to adopt first-class habits.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “The Science of Self-Realization”, Page 16″