Magic

The human body has a remarkable ability to respond to invisible influences. For instance, let's say you stay out in the sun too long. Next day, your skin will be red, or if you're lucky, tan. Your skin, without your knowledge or consent, allows melanin to move to the surface of the skin in response to the ultraviolet invasion. Or, say, you are walking along and trip a little. Your body responds to the mysterious force of gravity in a very complicated way, starting with the movement of fluid in the inner ear, and you catch yourself.
Now, here's my point: any feelable force in the universe will be reflected in some bodily function that deals with that force. Take the magnetic flux that moves from pole to pole. Humans don't respond to it much, as far as we know, but birds do, and there are neurological indications of how they do it.
So, if there is a force, however mysterious, that acts on humans and that influences our survival, we will have developed a mechanism for dealing with it. The key word here is mechanism. We had melanin long before we learned about the ultraviolet. Much of our entire body mechanism is geared to coping with gravity (jellyfish don't have an inner ear). Bugs have a wonderful six-legged gait that keeps them on an even keel.
So, what about ki, mana, and other posited forces of nature? I believe that if they exist there is a discoverable, chartable, testable mechanism that responds to them. No mechanism; they don't exist. That's my story and I'm sticking to it.