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 few 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 mor karma-bandha-phāṅsa (Gaurā Pahu 3, from Prārthanā): "I gave up sat-saṅga, oṁ tat sat, spiritual society. I associated myself with the material society. Therefore I am now entangled by karma, one after another, one after another."
So spiritual realization is difficult for person 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.
That information is given in the Bhagavad-gītā: avināśi tu tad viddhi yena sarvam idaṁ tatam (BG 2.17). Yena sarvam idam. This body is perishable, asat. But there is another sat, means permanent thing. What is that? Avināśi tu tad viddhi. You try to understand that thing which is eternal. What is that? Yena sarvam idaṁ tatam, that which is spreading all over your body. You pinch your body, you feel pain. Why? Because there is consciousness. Therefore consciousness is permanent. And as soon as the consciousness is gone, you chopped up your hand, no response. So that . . . it is a very nice statement. Tat, that consciousness, is avināśi, is eternal. Where is the difficulty?