If separation was real, how could you read these words that I am writing? How could you perceive the cloudy sky? How could you know seeing, feeling, sensing, perceiving?
The belief in a separate subject and object veils the clarity of our total intimacy and disregards the intimacy of experience. 
Separation is the initial assumption that infects your understanding and perception. 
Assuming a personal doer, you perceive an external world and an internal mind.