> The "burden of proof" is on both sides.

This is a common refrain from folks, who either accidentally or on purpose, are attempting to throw up barriers to have their beliefs genuinely challenged, and it works by misconstruing what "proof" actually means in a practical sense.

Here's a real world example of proof. A company wants to sell aspirin. To sell aspirin in the US, the FDA requires proof that the pills do contain a certain amount of aspirin, and that the pills don't contain any materials in high enough concentrations to be considered dangerous. Thus the company must prove a positive (Condition A: pills contain aspirin) and a negative (Condition B: pills don't contain harmful materials in harmful concentrations). For a single pill, both of these conditions are straightforward to prove. For condition A, grind up a pill and using methods of physical chemistry, quantify the concentration of aspirin and verify that the concentration is as it should be. Condition B is more arduous to prove, but still straightforward. You can either test for all harmful materials known to man or you can can attempt to quantify all materials present in the pill and show by exhaustion that there's absolutely no plutonium, arsenic or botulism in the pill (among millions of other things). Whew! Mission accomplished!

But hold on, it was a lot of work just to prove condition B for one pill, maybe we can simplify things a bit as there are approximately 1 billion other pills to test. So lets skip plutonium testing. It's a rare material that is not involved in any way in our manufacturing process. Since we don't have live cultures of any sort either, we can probably safely skip botulism. In fact, we can skip testing for millions of compounds because there's no probably mechanism for them to be introduced into our pills. "But wait," someone says from the back of the room, "How do we know for 100 percent certainty that these things aren't in our pills? Can we really be sure?" After reviewing the physical security of our facilities and double checking our process, we express our confidence in the new process. "But wait," that persistent guy in the back of the room says, "what if there are aliens from another planet with the power of teleportation who are spiking our pills with botulism? What now?" After recovering our senses, we kick that guy out of the room.

Cool, we now have a practical test process for ensuring that a pill is safe and effective. We just need to grind up each of our 1 billion pills, test them all, and we can be certain of the safety and efficacy of each of the pills we just destroyed. Hmm, we may have a problem. How exactly are we supposed to sell pills if our testing process is destroying them all? We closely monitor our manufacturing process, and we test the pills from a test run to ensure that our mixing process is sufficiently thorough to guarantee an even distribution of ingredients in each pill. Now instead of testing every pill we make, we can simply take a few from each batch. If there's something wrong or weird about the pills we test, we can test more to confirm there's a problem. "But wait," the persistent guy in the back of the room says. "What if teleporting aliens from another planet are swapping out the faulty pills in our test samples with good pills in order to trick us into thinking our batches are safe?" Wait, how did that guy get back in the room? Maybe it really was teleporting aliens. Whoops, nope. Someone left the side door open. That makes way more sense.

The problem of asking for proof in this situation is that proof has been provided, and in response you've escalated the request for proof without sufficient justification. We have many examples of folks demonstrating the techniques used to create crop circles, including the techniques necessary for shaping the crops and the techniques used for laying out a pattern. No crop circle presented has ever had qualities that are outside the possibility of the shown techniques, or advancements in those techniques. So trying to drag aliens back into the conversation is no different than insisting we test for teleporting aliens when we manufacture aspirin. If we want to bring aliens back into the subject of crop circles, we first need to prove 1) that aliens exist, 2) that they're visiting the Earth, 3) their methods of propulsion are totally outside our knowledge of the physical universe, and 4) that we have some reason to believe that they use grain fields (for example, why don't they use wild prairies?). If you can present that information, then we can talk about aliens and crop circles.