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an idea or a belief or an outlook that is based on an observation or an experience



 the sun must be hot because everything always warms up when the sun is shining


 the earth must be round because when the lower part of a ship disappears beyond the horizon the tallest part of the ship is still visible


 evolution and devolution must be happening because all newly born beings aren't totally dependant on the their parents but human babies are


 relativity theory must be right because when an atomic clock in an aeroplane is compared to the time of an atomic clock on the ground the clock in the aeroplane runs slower than the clock on the ground


quantum theory must be right because it explains atomic behaviour and it can be used to make precision devices like computers and tv's