 Sensations are appearances; their causes are not, but because we are bound to think of them in the guise of sensations, philosophical analysis is needed to pry them apart. The former are inner and the latter outer. Objects of perception must remain unknown in the manner in which sensations are known. Because a sensation is essentially an appearance to consciousness, it cannot be like anything other than a sensation in essential respects. However, Shepherd insists that sensations have inessential properties that are shared by beings of other sorts, such as variety (EASP: 163–4). There must be as much variety among the causes of sensations as there is among the sensations; otherwise CPI would fail. So although  sensations can be intrinsically like nothing but sensations, relations among sensations must have a likeness to relations among their causes. 