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reporters of the world around him. What if he were mad, or dreaming, or being deceived by
an evil demon? What if the world as he perceived it didn’t really exist? Perhaps he, himself,
did not exist! Eventually, he discovered a way out of this logical trap:I have convinced myself that there is absolutely nothing in the world, no sky, no earth,
no minds, no bodies. Does it now follow that I, too, do not exist? No. If I convinced
myself of something (or thought anything at all), then I certainly existed. But there
is a deceiver of supreme power and cunning who deliberately and constantly deceives
me. In that case, I, too, undoubtedly exist, if he deceives me; and let him deceive me as
much as he can, he will never bring it about that I am nothing, so long as I think that
I am something. So, after considering everything very thoroughly, I must finally con-
clude that the proposition, I am, I exist, is necessarily true whenever it is put forward
by me or conceived in my mind.^5Descartes had proposed a mechanical model for the universe and considered the bod-
ies of plants and animals to be living machines accessible to science— all except the mind.
The nervous system, according to Descartes, was merely a system of hollow tubes in which
ghostly “animal spirits” wafted throughout the body. The mind and soul were immaterial
and therefore not explainable by physical laws.
The French physician Julien Offray de La Mettrie saw no logical reason to invoke mysti-
cal animal spirits to explain mental processes. As a teenager, he had abandoned Catholicism
to become a Jansenist, the French version of the Protestant reform movement. His first
book, The Natural History of the Soul, argued that if humans had souls, then other animals
and even plants must have them as well because of the uniform laws of nature. The French
Parliament ordered all copies of his book burned in 1746, and Mettrie fled to the University
of Leiden, where he had previously studied medicine under Hermann Boerhaave, Vaillant’s
former mentor.
Under the more tolerant auspices of the Dutch, Mettrie penned an even more daring,
openly atheistic treatise, The Machine Man (1 747) ,^6 which challenged the immaterial-
ity of the mind and represented the first modern statement of the materialist philosophy.
The mind is not a tabula rasa at birth, as John Locke^7 had suggested— which implies that
human beings are both perfectible and malleable— but rather possesses an inherited range
of potential behaviors, compulsions, and limitations predetermined by inborn anatomical
and neurological factors. In other words, in animals as well as people, mind and soul are
inseparable from the brain and nervous system, which Mettrie speculated may possess prop-
erties “on a par with electricity.” Over two hundred years before the biolinguistic theories of
Noam Chomsky, Mettrie asserted that the primary mental process that separates humans
from chimpanzees is the capacity for language.^8
The arguments in The Machine Man were too radical even for liberal Holland, and the
printer was ordered to deliver his entire stock to the Consistory of the Church of Leiden
for incineration. Once more Mettrie was forced to flee, this time to Berlin where Frederick
the Great of Prussia, an admirer and patron of Enlightenment philosophers, offered him
protection and a pension. He immediately turned his attention to plants.
Written in a light- hearted, chatty style during his stay in Berlin between 1748 and 1751, The
Plant Man articulated Mettrie’s belief in the uniformity of life.^9 Poets had been doing this