 External existence contrasts with internal existence. Shepherd has a layered definition of this pair of notions. In the first instance, internal existence pertains to the capacity for sensation in general, that is the mind, or power of thought and feeling. External existence pertains to the exciting cause of any particular sensation (EASP: 40). In context of CP, nothing that satisfies one of these definitions can satisfy the other. A mixture of both is necessary for the existence of a sensation. Now sensations are nothing other than appearances in consciousness, realizations of the general capacity for sensations, i.e., the mind. With this in view, Shepherd advances the thick notion of internal existence: an entity is thickly internal just in case one and only individual can be conscious of it. A thing is thickly external, then, just in case it can be perceived by more than one human being. The thick distinction marks the difference between what is private and what is public.