The Hydraulic Model of Brain Functioning

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This idea is often attributed to the famous physicial Galen, who also developed the idea of the "four humors", who worked in the second century, although it can be traced back to early Greek philosophers. If you want to read translations of Galen's writings look here.

The idea was highly developed by Descartes, as part of his mind/brain studies. To read about Descartes' overall contribution to the mind/brain "problem" go here.