Mind, Brain, Monism, Dualism
Philosophy of Mind: A Short Introduction (Oneworld Press: 2005), p. 136.
More to the point, brain processes, composed as they are of meaningless chemical components, seem as inherently devoid of intentionality as soundwaves or ink marks. Any intentionality they have would also have to be derived from something else. But if anything physical would be devoid of intrinsic intentionality, whatever does have intrinsic intentionality would thereby have to be non-physical. Since the mind is the source of the intentionality of physical entities like sentences and pictures, and doesn't get its intentionality from anything else (there's no one "using" our minds to convey meaning) it seems to follow that the mind has intrinsic intentionality, and thus is non-physical.