"
When I was a child
I caught a fleeting glimpse
Out of the corner of my eye
I turned to look but it was gone
I cannot put my finger on it now
The child is grown
The dream is gone
I... Have become comfortably numb
This verse is explaining before he took the drug and when he was a kid he saw something out of the corner of his eye that was very weird but when he looks, there is nothing there. He doesn't understand why but, that was all a dream and that now he did this, he is comfortably numb. This is an example of an illusion when he sees something that really wasn't there or that it real in the first place.