The concept of need is often looked upon rather unfavorably by economists, in contrast with the concept of demand. Both, however, have their own strengths and weaknesses. The need concept is criticized as being too mechanical, as denying the autonomy and individuality of the human person, and as implying that the human being is a machine which "needs" fuel in the shape of food, engine dope in the shape of medicine, and spare parts provided by the surgeon.