 On Shepherd’s theory, we perceive particular things by sense because they affect the sensory organs and thereby produce sensations that simultaneously begin to appear in consciousness. This prompts the mind to implement “latent reasoning”: this sensation began to exist; it must have a cause; so there is something that is its cause. Reason need not rehearse this argument, but it attaches to the sensation an idea that (purportedly) represents a particular thing that causes the sensation. It is believed to exist, but only its effect appears in consciousness. To be clear, the state in which a person perceives an object by sense comprises several things: (1) an object, or mass of qualities which can be perceived by sense, (2) the mixture of the object with the mind and sensory organs of a perceiver, (3) a sensation caused by this mixture which simultaneously begins to appear in awareness, (4) an idea produced by reason which (purportedly) represents the cause of the sensation and belief that the cause exists. That said, a mass of sensible qualities is known only as the cause of sensations which appear in consciousness. We are acquainted with nothing but the sensations and have only an idea that refers to something as its cause. So when sensible qualities are not perceived and considered in abstraction from their effects on our senses, they are “unknown powers or qualities” (ERCE: 46–7); “the positive nature and essence of unperceived beings cannot be known” (EASP: 243). In sum, an operation of reason prompted by a sensation that begins to exist in consciousness generates belief that the sensation is caused by something else. 