One of the famous aphorism of Vedanta school of philosophy is - Only universal consciousness is the Truth. World of duality is a myth.
Word 'duality' is a very important and interesting word.
Is there anything called 'darkness'?
There is light we know. But, darkness?
Absence of light is darkness.
If light is zero percent, then it is pitch dark.
If light is say, 30% present, it is semi dark and so on.
So we are looking at a continuous scale and not a binary this or that.
Why do we say someone is absolutely bad? How about saying he is zero percent good?
If you look around, most of the things as we perceive in this world are interpreted like this. Good or bad. Like or dislike. Love or hate. So on and so forth.
It's really not an intentional mistake. We have something called intellect. It serves a very important role. It has to process so much information and help you make decisions. There is no way a poor tool like intellect can view life as a continuum and help you make decisions. Intellect has to quickly draw upon its reservoir of past experiences and provide you with least number of options. So the intellect categorizes everything into two neat little piles. So it is possible that 0% light is treated same as 49% light. Both go into the category labeled as 'dark' and everything else in the category named light.
You can take this example to any other attribute also. Good and bad etc.
This way of categorizing helps us survive in this world. This is what is required when you have to make fast decisions with limited data.
On the flip side, this also increases separateness. We can convince ourselves in our mind that everything in this universe is nothing but pure consciousness. We only see different manifestations of the same self. Easy to say but with our intellect it is hard to see unity in life because it is busy categorizing all the time.
Without seeing unity, we are going to be very unhappy in the world of separateness. You are a bad person. I am a good person. If you like me, you are good person. If you do not, you are a bad person. You are a hard working person. He is a lazy person. So and so forth.
Separateness increases insecurity because it generate us versus them and keeps increasing it. Vicious circle.
Swami Vivekananda said - there is nothing called falsity. It is on the same scale as truth. Untruth is nothing but lesser truth.
If you start consciously viewing life in this kind of a continuous spectrum of varying degrees, it slowly starts reducing the separateness.
Ultimately we will realize that all the variation is due to our own projection on the pure consciousness. It's like we may find a mirror dusty but it is not uniformly dusty. In some places it is full of dust. In some places, we can see somewhat but blurry. In some places it is clean. We can sit and fret about or just clean it off and use it properly. Seeing unity is like seeing into a clean mirror. It takes effort to get there. In the mean time knowing the theory helps till we perfect the practice.