What is meant by, "the whole of a person is greater than and different from the sum of the parts"?

What this sentence means is that one aspect of someone's life is not greater than the whole body as a whole and that the summation of one's life make up the whole of a person.
The whole person is believed to be made up of many aspects: physical health and fitness, mental health, emotional health, spirituality, social life, environmental and occupation aspect, etc. 
In the health sector for instance, health providers had been advised not to treat people's diseases alone but to take cognizance of the person as a whole; the health aspect is just one of the life aspects of a patient.