This chapter argues that symbiosis is more important than has yet been appreciated. It points out that Margulis’ actual definition of symbiosis leaves quite a lot to be desired. The “living together in physical contact of different species…” is neither precise nor general enough. One needs something that allows for more indirect forms of interaction—a causal reading, rather than mere contact. This emphasis on causal interaction allows at least considering predator–prey relations as potentially symbiotic (not to mention pollinator to pollen–producer relations): Mutual inclusion in life cycles is the theme of the chapter. There is no presumption of a symmetrically beneficial relation. Symbiosis, on this account, can run all the way from straight-out parasitism to obligate mutualism or even fusion into a new single species. Further, this discussion proposes that the difference between mutual and parasitic relations can be expressed thermodynamically, in terms of partners adding to each other’s free energy. It may be hard to get into the mutualistic regime, but once there, selection for cooperative interactions can be very strong. Thermodynamically, stable (rich) dissipative structures can be favored strongly when there is a generous flow of available free energy and physical conditions are benign. Additionally, it considers cancer as a vivid example of the breakdown of mutualism: Obviously, rogue cells can be selected for in the local environment that their success eventually destroys. Then the discussion turns to a view of humans as parasites on the earth’s ecology that is all too convincing and claims human race needs cultural evolution to reshape behavior into a form that will allow a sustainable interaction between human and the living environment.