Galileo, in 1623, put forward the view that science should only be concerned with primary qualities, those of the external world that could be measured or weighed. So-called secondary qualities, such as love, beauty, meaning and value, were said to lie outside the realm of science. Descartes, himself, supported this idea and proposed two categories: mind and matter. The matter category related to physical or extended substance, the mind category to thinking substance - that which is unextended and indivisible.
These philosophers and scientists thus distinguished the physical operation of the brain from the thought process. While the former was thought to be amenable to scientific study, consciousness was excluded from the scientific world-view. It is only recently that researchers have begun to challenge this mind/matter split with evidence that many human qualities traditionally associated with the mind, such as personality, are, at least in part, determined by biochemistry. Some researchers believe that consciousness itself may emerge as a by-product of the complex workings of the human brain.
A major impetus to the study of the physical workings of the brain came in 1791, when Galvani showed that electricity existed as a force within the body - in fact, inside the brain cells. He showed in a sequence of experiments that it was possible to control the motor nerves of frogs using electrical currents. (The novel "Frankenstein" by Mary Shelley is testament to the flurry of public interest in this new research.) However, Galvani did not have the technology to measure the currents involved in the body; they were too small. His experiments were later confirmed by Du Bois-Reymond in 1850, who found that neurons emit pulses of electricity that travel at around 200 mph. Purkinje, in 1838, found that nerve cells consist of two parts: a nucleus similar to other cells and a set of fibers which emanated out from the nucleus - these were later identified as the axons and dendrites. In 1870, Golgi made the observation that there were literally billions of neurons making up the central nervous system and established that the neurons in the brain sent information to the motor nerves and that information from the sensory nerves was sent to the brain for analysis.
In the early 1900's, Adrian, Gasser and Erianger found that the electrical pulses within the neurons caused chemicals to be released, the function of which was to send a message to other neurons using the connections between them and that it took one-thousandth of a second for the neuron to recharge after this firing process had taken place.
These initial discoveries paved the way for modern neuroscience which in recent years has yielded enormous amounts of information about the physical functions of the brain.