By nature living entities have particular types of body and particular types of psychic and biological activities accordingly. There are four classes of men functioning in the three material modes of nature. Those who are purely in the mode of goodness are called brahmanas. Those who are purely in the mode of passion are called ksatriyas. Those who are in the modes of both passion and ignorance are called vaisyas. Those who are completely in ignorance are called sudras. And those who are less than that are animals or animal life. However, these designations are not permanent. I may either be a brahmana, ksatriya, vaisya or whateverin any case, this life is temporary. But although life is temporary and we do not know what we are going to be in the next life, still, by the spell of this illusory energy, we consider ourselves in the light of this bodily conception of life, and we thus think that we are American, Indian, Russian or brahmana, Hindu, Muslim, etc. And if we become entangled with the modes of material nature, then we forget the Supreme Personality of Godhead who is behind all these modes. So Lord Krsna says that men, deluded by these three modes of nature, do not understand that behind the material background is the Supreme Godhead.