SOUL (psuche):
1) breath
1a) the breath of life
1a1) the vital force which animates the body and shows itself in breathing
1a1a) of animals
1a12) of men
1b) life
1c) that in which there is life
1c1) a living being, a living soul
2) the soul
2a) the seat of the feelings, desires, affections, aversions (our heart, soul etc.)
2b) the (human) soul in so far as it is constituted that by the right use of the aids offered it by God it can attain its highest end and secure eternal blessedness, the soul regarded as a moral being designed for everlasting life
2c) the soul as an essence which differs from the body and is not dissolved by death....
SPIRIT (pneuma)
2) the spirit, i.e. the vital principal by which the body is animated
2a) the rational spirit, the power by which the human being feels, thinks, decides
2b) the soul
3) a spirit, i.e. a simple essence, devoid of all or at least all grosser matter, and possessed of the power of knowing, desiring, deciding, and acting
3a) a life giving spirit
3b) a human soul that has left the body
4) the disposition or influence which fills and governs the soul of any one
4a) the efficient source of any power, affection, emotion, desire, etc.(14)
Thus in Greek "soul" refers to the animating principle which feels, desires, and can attain everlasting life with God. And "spirit" is also the animating principle which feels, thinks, and decides. And notice once again, the use of the word soul to define spirit (twice in fact: 2b,3b). Only #4 for spirit gives so much as a hint the two might be distinct.
Summing up, overall the definitions of the English words and lexical entries for the Hebrew and Greek words indicate that "soul" and "spirit" are interchangeable terms, with common characteristics ascribed to both.
http://www.dtl.org/dtl/treatise/soul-spirit-1.htm