Once again…

A mathematician, engineer, and doctor walks into a first grade classroom. One of the students asks "What is 2+2?"
The mathematician, after a period of though, replies "Through the development of number theory and logical reasoning over thousands of years, we can conclude that 2+2 is equal to 4."
The engineer, after typing the equation into his trusty calculator, replies "Thanks to modern technology, we can conclude that 2+2 is equal to 4."
The doctor, not wasting a moment to think, replies "4".
The mathematician and engineer both look at the doctor and asks "How do you know?"
The doctor shrugs and says "I memorized it."