According to Hippocrates, four internal liquids, called humors were responsible for the health and mental stability of every individual. These liquids were blood, black bile, yellow bile, and phlegm. According to Hippocrates when these fluids were in balance, the human body and mind perform in perfect harmony. But when there was an imbalance in these fluids, the dominant one creates an overload of temperament. This was known as the theory of the four humors.
However, today we know more about how the human body works. More specifically, we know about how neurons work. For instance, we know that it is not the body fluids that influence behavior, but it is the neurotransmitters that cause changes in personality, mood and behavior. For example the neurotransmitter Serotonin affects mood and hunger. Thus, changes in mood and behavior depend on the kind and strength of an action potential sent through an axon.
Apparently, Hippocrates had underestimated the power of the neurons. However we must not forget that at the time when he said this, there were limited resources and tools available for use. It is only after the microscope was invented that we discovered the neuron for the first time.
In a way Hippocrates was correct when he connected that mood and behavior were influenced by substances in the body; however he specified the wrong substances. This is the reason why we refute his theory of the four humors.