 Philosophers who endorse externalism commonly also endorse Humeanism, though it is important to remember that many internalists, including most noncognitivists and expressivists about moral judgment, also accept the Humean theory of motivation. Indeed, some contend that the basic observation that supports externalism also lends support to the Humean theory: wide variation in the motivational impact of moral judgments suggests not only that they motivate contingently but that they do so via some conative state. Still, externalists need not be Humeans. Shafer-Landau, who rejects both Humeanism and internalism, holds that moral beliefs are indeed intrinsically motivating—they can motivate by themselves. But contra internalism, they are not necessarily motivating. Intrinsically motivating beliefs may fail to motivate under conditions of extreme exhaustion, serious depression, or overwhelming contrary impulses (Shafer-Landau 2003, 147–148). The fact that Shafer-Landau treats the defeasibility of moral motivation under such conditions as supporting a form of externalism, whereas Smith treats defeasibility under like conditions as compatible with a form of internalism, suggests some disagreement among philosophers as to precisely when a view should be classified as a form of internalism or  externalism.[10]