What if Bigfoot is dressed in a burka? Under your assumptions your system is logically sound and under the system you can conclude, but in broader context you a

What if Bigfoot is dressed in a burka? Under your assumptions your system is logically sound and under the system you can conclude, but in broader context you assume that which you would like to prove. You started with no one saw bigfoot, and add it as a necessary condition for bigfoot's existence. It's not a necessary condition. If your assumptions are false, then so is your conclusion, in the end.