Maybe you don’t understand how complex a structure the human brain is. Believe me, it makes the sidereal universe look like a child’s building set. There are many times more possible inter-neuronic connections than there are atoms in the entire cosmos—the factor is something like ten to the power of several million.
It’s not surprising that a slight change in electrochemistry—too slight to make any any important difference in the body—will change the whole nature of the mind. Look what a little dope or alcohol will do.
Poul Anderson, Brain Wave (1954)