Originally Posted by Captain Stable
The Mark I eyeball remains your best friend and collision avoidance tool.
You just need to fly with any kind of radar service, or TCAS to realise the limitations of the Mark 1 eyeball. Even when you know where to look, it's hard to spot other aircraft, and if one is ascending or descending into your path from behind, with its wings obscuring you, the eyeball has no chance. In my opinion, it mainly works because of the big sky theory.