From the Angelic Doctor...
"The ancients, not properly realizing the force of intelligence, and failing to make a proper distinction between sense and intellect, thought that nothing existed in the world but what could be apprehended by sense and imagination. And because bodies alone fall under imagination, they supposed that no being existed except bodies. Thence came the error of the Sadducees, who said there was no spirit."
-- Saint Thomas Aquinas (1225-74)