ts imprecision, asserting that Lovelock had not presented one particular theory, but rather four separate, but closely linked, hypotheses:
- Co-evolutionary Gaia — that life and the environment evolve in a coupled way, which Kirchner claims was not new and already accepted scientifically.
- Homeostatic Gaia — that life maintained the stability of the natural environment, and that such stability allowed life to continue to exist.
- Geophysical Gaia — that the Gaia hypothesis generated interest in geophysical cycles, therefore leading to new research in terrestrial geophysical dynamics.
- Optimising Gaia — that Gaia shaped the planet in a way which made it an optimal environment for life, which Kirchner claims was not testable and therefore was pseudoscientific.
Furthermore, of homeostatic Gaia, Kirchner recognized only two alternatives: "weak Gaia", which asserts that life tends to make the environment stable for the flourishing of all life, and/or "strong Gaia" which asserts that life tends to make the environment stable in order to enable the flourishing of all life. Strong Gaia, Kirchner claims, is untestable and therefore pseudoscientific.
In Joseph Lawrence's book, Gaia: The Birth of an Idea, Lawrence argued that Kirchner's attacks were principally against Lovelock's integrity, and not against the hypothesis itself. Lovelock did not attack Kirchner's views until his autobiography Homage to Gaia, in which he dismissed Kirchner's position as sophistry. Lovelock, however, did attempt to disprove the claim that the theory is not scientific because it is impossible to test it by researching and developing a controlled experiment.
Lovelock was also careful to present a version of the Gaia hypothesis which had no claim that the Earth's biological systems intentionally or consciously maintained the complex balance in the environment that life needed to survive. Instead, it appears that the claim that Earth acts intentionally was a metaphoric statement and was not meant to be taken literally. This restatement of the Gaia hypothesis was more acceptable to the scientific community, although, at this point, pretty much devoid of meaning.
 Enduring scientific criticism
Most of the systems used as examples or explanations to support this "hypothesis" make no sense in regard to the reality of how life has evolved and the environment has changed.
While, among other factors, life on Earth certainly alters the environment, the life that then is seen thriving evolved and adapted to do so, thus there is no magical mechanism that endlessly returns conditions to some "ideal" balance. This is easiest to see over longer terms - before plants started photosynthesizing, there was no free oxygen in the atmosphere - and when they did, there was a mass extinction of creatures to whom it was poisonous.
Therefore, many forms of life that survive in some niches can either be wiped out, or become much more successful, due to "life-made" or geologic or astrophysical changes to the environment.
Stephen Jay Gould criticized (and other scientists still criticize) Gaia hypothesis as merely an attempt to re-explain processes already explained by evolution, natural selection, and adaptation. Gould said that, in order for the hypothesis to hold, either of the following would have to be the case:
- Evolution processes that are guided by some "magical force," also known as "guided evolution", which is already rejected by the scientific community as being unverifiable and, thus, pseudoscience.
- Evolution processes that are guided by a system of adaptation, natural selection, and evolution. Wait! Science already accepts this as something else, something that needs no further explanation.
Gould then went further to dismiss it as "merely a metaphor for naturally occurring processes (referring to evolution).
Also, according to most scientific critics, especially Dawkins and Ford Doolittle, the Gaia hypothesis requires an ecological foresight and teleologism to be true, concepts that are not scientifically viable, and are considered about as scientific as intellig