Krishna says Sat (reality/permanence) never ceases to be and Asat (unreal/impermanence) has no existence. A Gyani is one who can distinguish between the two.
The rope and snake analogy is often quoted in many cultures to understand the intricacies of Sat and Asat . A man reached back home at dusk and found a snake coiled at the entrance to his home. But in reality it was a rope left by children, that looked like a snake in the semi-darkness. Here the rope signifies Sat and the snake, Asat . Until he realises Sat i.e. the rope, he is likely to adopt many strategies to handle Asat i.e. imagined snake. He could attack it with a stick (fight), run away (flight) or he try to light a torch to check out the reality. The best strategies and skills would go in vain when our perception is that of Asat .
Asat derives its existence from Sat , just as the snake doesn't exist without the rope. Since Asat owes its existence to Sat , it can influence us like a nightmare which can make our body react as if it were sweating in sleep.
A litmus test given by Krishna to identify Asat is; that 'which didn't exist in the past and wouldn't be there in the future'. If we take the example of sensual pleasure, it wasn't there before and wouldn't be there after some time. The same is the case with pain and for that matter all polarities. The indication is that Asat exists in time whereas Sat is eternal.
The Sat is the inner self which is eternal and Ahankaar is Asat which sustains itself with the support of the inner self. The day we discover our inner self (rope), the Ahankaar (snake) automatically disappears.
Source - Daily World