1) Christopher Columbus claimed to have seen three mermaids on his voyage to America in the 1440s, but whether he actually saw them or imagined them is unclear.
2) Our perceptions are influenced by our personality, emotions, culture, and prior experiences or biases. What we see is not always objective reality.
3) The essay discusses how sensory perception can deceive the brain in limited ways biologically, superficially, and through customs or preconceptions. Our perceptions are shaped by both internal and external factors.