<Dl> <Dd>--Leibniz, Monadology </Dd> </Dl> <Dd>--Leibniz, Monadology </Dd> <P> Doubt about the possibility of a mechanistic explanation of thought drove René Descartes, and most other philosophers along with him, to dualism: the belief that the mind is to some degree independent of the brain . There has always, however, been a strong argument in the opposite direction . There is clear empirical evidence that physical manipulations of, or injuries to, the brain (for example by drugs or by lesions, respectively) can affect the mind in potent and intimate ways . In the 19th century, the case of Phineas Gage, a railway worker who was injured by a stout iron rod passing through his brain, convinced both researchers and the public that cognitive functions were localised in the brain . Following this line of thinking, a large body of empirical evidence for a close relationship between brain activity and mental activity has led most neuroscientists and contemporary philosophers to be materialists, believing that mental phenomena are ultimately the result of, or reducible to, physical phenomena . </P> <P> The size of the brain and a person's intelligence are not strongly related . Studies tend to indicate small to moderate correlations (averaging around 0.3 to 0.4) between brain volume and IQ . The most consistent associations are observed within the frontal, temporal, and parietal lobes, the hippocampi, and the cerebellum, but these only account for a relatively small amount of variance in IQ, which itself has only a partial relationship to general intelligence and real - world performance . </P>

Which is the smallest part of the brain