Doctor or Engineer?

What's the difference between a doctor and an engineer? I got the opportunity to find out the answer to this question in Hospital Kuala Lumpur, when a dude, a future medic student, gave a speech for a closing ceremony...

There was once an engineer, he was fixing a car in his garage when suddenly, he saw a doctor passing by. so, he called the doctor over and started a conversation...

Engineer : Hey doc, we practically do the same thing, I mean... we cut open the body (the car), we take out the heart (the engine), we fix it, and then we place it back into the body...
Doctor : Yes, u're right.
Engineer : So, tell me something, why are u more popular and richer than me? if i'm right, then we should be equal... don't u think?
Doctor : Try fixing it while the engine is on...