No. Not must. substitute "is assumed to be, as a working hypothesis" It is the only place to start. With the known. If you do not start there, Yazata, I ask you in all seriousness: what is to prevent you from starting with angels, aliens or ghosts or magic? You, Yazata, have a preexisting bias against angels aliens, ghosts and magic, otherwise every time you look out the window and say "what the heck is that furless creature" you would give as much weight to "it might be a garden faery" as to "it might be squirrel with mange". You have asserted this idea that pre-existing bias is bad so many times, I'm calling you on it. Do you look into your garden and think "faery" as often as you think "funny-lookin' squirrel"? Though the example is a bit tongue-in-cheek, it is a serious question. I want to understand your logic that a bias in favour of things that we know already exist is bad. I plan to stop the ridicule and insults themselves, but be sure to read and address the question at the bottom of this thread, above) .