It is most difficult to see things that are not there but the obvious is even harder to observe.
Do fish notice water? No, it is all around them. Is water important for fish? Certainly, it is. This common difficulty to notice the evident makes detecting the obvious a vital art of masters: therefore, learn to marvel why some long held belief is so certain and look at it otherwise. Such trifles can change the world.
This reminds me of a story:
It used to happen when Nasreddin was still forever young and his beard was still black and cheeky, before his pilgrimage to the holy city of Mecca.
Season after season, day in day out and even three or four times a day, he would ride his donkey through the toll gate up the valley. So often that you couldn't tell anymore whether he was coming or going, annoying like a buzzing fly. Time after time the customs officers would check his load, saddlebags and even his turban.
They never found more than loads of hay. They knew there had to be something but just couldn't find what. Finally, they gave up.
This ploy continued under the nose of the border guards, for several years, to the annoyance of the captain.
Forty years later, the retired captain leaned over the table at the tea house in Aksehir, closer to the Mullah's now respectable white beard and asked:
"Just tell me Hoça, before we both die in our old age and your smart trick gets lost. What were you hiding? I checked you so many times, with my own hand, and your donkeys never carried anything other than hay. What was it that you smuggled?"
The Mullah stared at him with his round innocent eyes.
"Donkeys, what else my good man?"