there is neither self nor other-than-self.
To come directly into harmony with this reality
just say when doubt rises "not two".
In this "not two" nothing is separate,
nothing is excluded.
No matter when or where,
enlightenment means entering this truth.
And this truth is beyond extension
or diminution in time and space:
In it a single thought is ten thousand years.
Emptiness here, emptiness there,
but the infinite universe
stands always before your eyes.
Infinitely large and infinitely small;
no difference, for definitions have vanished
and no boundaries are seen.
So too with Being and non-Being.
Don’t waste time in doubts and arguments
That have nothing to do with this.
Even though we may have experienced Mirror Like Samadhi and have had a deep realization of Non-Duality we humans have to function in this world which takes thought and may lead to confusion so Seng Tsan gives us a simple practice to remind us of the clarity and understanding we have experienced. Say not two and with this reminder we can bring our thoughts and actions into harmony with this understanding. This is a simple practice which I have found useful in situations which could otherwise be tense and emotionally confusing.
It takes year of practice to learn to bring with consistency the experience and understanding of deep samadhi off the cushion and into our daily lives. And first we have to experience deep samadhi and the resulting experience which also can take years. As I was told by one of my teachers life is too short to completely polish the mind, but really it doesn't matter this is the nature of being a human. We just keep on practicing. It is possible with lots of time and effort that we may experience a harmony between thought action and understanding in most of our lives. It is possible to function without ego and to see the world through the eyes of non-duality.