TCM was very well developed more than 2000 years ago. People who have studied the classics of TCM understand that TCM books written in that era is like a bible to our practice. But why is it so different from modern medicine?
I really like the way how Andy Lee, L.Ac. explained it. Imagine you were an alien from another planet. Your mission is to come to earth and study how human being's body functions. What is your approach?
1. You take one human being and cut it in half and study it. Then cut the half in another half and study it. Then cut the quarter in half into eighth. And keep cutting to infinite small pieces until you can't cut it anymore and study that.
or
2. You don't cut a human being in half because you realize if you do that, you would kill the human being and a dead human being behaves and functions differently from a live human being. So you observe their daily lives. When human beings are in a cold environment without enough clothes, they sneeze and have discharge coming out of their noses, or their body temperature rises but wants to put on more clothing.
First approach is of our modern medicine, isolating our body into many different systems of their own and study under a microscope. Second approach is of Traditional Chinese Medicine, a holistic approach or what we called "Black Box Theory" nowadays.
Shang Han Lun (Treatise on Cold Damage) written 2000 years ago already had a very complete picture of what happens when cold "evil" enters human body.