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I would like to thank John Gilbert (Letters, 15?August) for his kind words about my book On Gaia (reviewed by Jon Turney, 8?August). However, I would like to clarify that I do not make a ¡°category error¡±. Gilbert suggests that I?conflate Gaia theory, which he uses as the name for a field of scientific enquiry, with the Gaia hypothesis, which is the proposal that Earth¡¯s environment has been moulded by life to help to keep it comfortable and stable for life over Earth¡¯s long history.

The field of scientific enquiry referred to by Gilbert does indeed exist, known to most as Earth system science. In my book, I?scrutinise the Gaia hypothesis only. I?do not refer to Gaia theory at all (the term never appears, except in quotations from other works). My book makes no claims to evaluate the field of Earth system science, although hopefully it advances our understanding in this area.

Toby Tyrrell
Professor of Earth system science
University of Southampton