I asked because someone told me that you couldn't start the engines without running the pumps. That didn't seem likely since the engines have their own pumps, and (at least in aircraft like a 737 or 747) the fuel tanks are presumably above the engine pumps and thus would drain by gravity into the pumps as long as the aircraft is straight and level. I believed that you could start and run the engines but that they might not perform ideally in all situations without the additional pressure from the other pumps in and around the tanks. From what everyone has said here, it looks I was right and he was wrong.