An Engineer, Scientist And Mathematician On A Train

An engineer, scientist and mathematician were riding on a train through Kansas when they spotted a cow similar to the following:

The engineer said, "Look! All the cows in Kansas are black!".

The scientist then corrected the engineer, "No! Some cows are black."

The mathematician then corrected the scientist, "In Kansas, there is at least one cow, at least one side of which appears to be black."