The contemporary error is to fail to understand the close intertwining of the intellect and the will, and thus to mistake thoughts for volitions:
This is how Hare looks at practical thinking, and hence why he calls its conclusions imperatives. He also calls these imperatives volitions, but this is a mistake. The imperative is an act of thought, but the act of volition belongs to the will. These two acts do, indeed, necessarily go together, but they are different acts and belong to different faculties. Aquinas speaks more correctly when he says that to command is an act of reason which presupposes an act of will (he also says, significantly, that an ‘ought’-judgement is an indicative way of expressing a command; ST: Ia IIae, q17 a1).
As has been pointed out ad nauseum, Foot emphatically rejects these subjectivist approaches in her later work (cf. the post here and the one immediately following).