Engineers think that equations approximate the real world.
Scientists think that the real world approximates equations.
A Mathematician, a Biologist and a Physicist are sitting in a street cafe watching people going in and coming out of the house on the other side of the street. First they see two people going into the house. After a while they notice three persons coming out of the house. An engineer, a physicist, and a mathematician are shown a pasture with a herd of sheep, and told to put them inside the smallest possible amount of fence. The engineer is first.