Who is asat? Asat means that does not exist, or that will not exist. There are two things: one thing will exist permanently, and one thing will not exist. It may exist for few minutes, or few hours or years. So this material world is asat, because it will not exist. Just like your this material body, it will not exist. Everyone knows. Everyone knows that it is born at a certain date, it will continue for certain years, it will produce some by-products, it will change into different forms, and then it will become old and then dwindle and one day finish. This is called ṣaḍ-vikāra, six kinds of changes. It is not progress. If one is progressing in his age, it is not progress. It means he is going to death. Suppose I am seventy-eight years old. So I have... Seventy-eight years I have already died. Only, say two or five years, or something like remaining balance. So people say "advanced age." No. Advanced in death, not advanced in age. So this is asat. It will not stay. It has begun to die from the very moment of his birth. If you ask a newly born child how old it is, the mother says, "It is one month old." So one month means he has already died one month. And balance months and years he has to die. Simply he has to wait for that death. So this kind of duration of life is called asat. And this kind of existence, the material existence, it also asat. Narottama dāsa Ṭhākura therefore sings, sat-saṅga chāḍi' kainu asate vilāsa, te-kāraṇe lāgila ye karma-bandha-phāṅsa. "I gave up sat-saṅga, oṁ tat sat, spiritual society. I associated myself with the material society. Therefore I am now entangled by karma by karma, one after another, one after another."
So spiritual realization is difficult for persons, asadbhiḥ. Why it is difficult? Viṣayātmabhiḥ. Because they are simply attracted by the four principles of material life: eating, sleeping, sex life and, one day, death. They cannot. One must be above this interest. One must be very sane man, that "These kinds of interests are there in the animals. So if I am also interested in only these things, then where is the difference between this dog and me?" Something more.