The Buddha defined the Fire element as “that by which one is warmed, ages, and is consumed, and that by which what is eaten … gets completely digested.” In other words the Fire element within is metabolism. It’s our energy.
So sitting in meditation we can experience the heat of the body, feel the cooler air we breath in contrast with the warmth of the air as it leaves the body, feel the heart pumping, and call to mind the myriad chemical combustions taking place at the cellular level, sparks of electricity in the muscles, nerves, and brain. All of this energy is borrowed from the Fire element outside of us.
The Fire element outside is the raw physical energy in the universe, from the nuclear fusion in the heart of the sun to warmth of a cup of coffee, from the molten core of our planet to the chemical energy stored in our food as fat, sugars, and proteins.
We feed the body by taking in the sun’s energy embodied in plants or (for those of us who aren’t vegetarians) flesh. We warm ourselves in the rays of the sun, either directly or through burning fossil fuels that grew in the sunlight of ages past.
And we have to keep replenishing the body’s fuel because the element Fire is forever leaving: radiating from our skin, wafting away on our exhaled air, lost in the warmth of our feces and urine. And so the Fire element, like Earth and Water, simply flows through us, unstoppable.
And as we observe the energy within the body, we can be aware that it’s actually another river — a river of energy — passing through this form, that it is really not ours at all. “This is not me. This is not mine. I am not this.”