  IIIp28, the proposition establishing Spinoza’s doctrine that human beings desire whatever will bring joy and are averse to whatever will lead to sadness, allows Spinoza to connect the objects of any human desire with joy or the avoidance of sadness. So, if it is true that we call a thing good only if we desire it, then it will also be true that anything we call good will be joy or what leads to it. Understood in this way, IIIp39s simply restates the doctrine of IIIp9s in light of IIIp28.