The argument went back and forth: the immortal soul was in the head and the mortal soul was in the heart (Plato) versus the soul as an entity which could not be divided and therefore could only reside in the heart (Aristotle). Following are two quotes from Heart by Gail Goodwin, which illustrates the evolution of Greek thought regarding the heart:
"The heart, the knot of the veins and the fountain of the blood which races through all the limbs, was set in the place of guard, so that when the might of passion was roused by reason warning that something was amiss due to an outside cause or to the inner lusts, [the heart] would signal to everything in the body and thus allow the principle of the best to have command over all of them." Plato (427–347 B.C.)
"For nature, when no other more important purpose stands in her way, places the more honorable part in the more honorable position; and the heart lies about the center of the body…" Aristotle (384–322 B.C.)
Godwin, G. (2001). Heart. New York: Harper Collins Publishers