Depression should be studied holistically

Chapter 10 endnote 33, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

Depression is an imbalance of many entwined parts of the nervous system that we can understand only by treating the whole person, not by treating one system in isolation like the parts of a machine. [...] In depression, dysregulation is widespread.

Depression is a psychiatric disease, a neurological disease, a metabolic disease, and an immunologic disease all rolled into one. In depression, there are genetic abnormalities related to cell development, cell maintenance, cell death. The abnormalities can be seen all over the brain, and in all neurotransmitter systems. Dysregulation is widespread in immune genes, circadian rhythm genes, in growth factor genes, and in genes that regulate epigenetic sensitivity.

This sort of widespread dysfunction is never seen in animal models of depression, because scientists usually opt for a reductionist approach to science. They study one system, or even one aspect of system, at a time, in isolation. This approach relies on Descartes' metaphor that the body, like a machine, can taken apart and studied one piece at a time.