Qi is "perceived functionally, by what it does," is critical to remember. Entities in Chinese Medicine are almost unanimously "perceived functionally," that is, defined by what they do. We'll see this affirmed again and again when we look at the meridian system. Meridians are defined by what they do, functionally - the organ system - organs are defined by what they do, functionally - and the Vital Substances of the body - which again are defined by what they do, functionally for the whole organism.