I think you've got it backwards. The default assumption is that only the natural world exists. If something is not understood, the assumption should be that it

I think you've got it backwards. The default assumption is that only the natural world exists. If something is not understood, the assumption should be that it has natural causes. A supernatural cause would imply the existence of something outside of natural causes, and to assert that that something exists without being able to demonstrate it is where the unfounded confidence lies.