1. By what means is it that we acquire the notion of continuous existences (as opposed to the interrupted sensations by means of which we know the things continuing to exist)?
2. By what means do we acquire the notion of external existences (as opposed to the internal existences of the mind)?
3. By what means do we acquire the notion of independent existences (as opposed to the existence of things that depend on our minds)?
These three questions are obviously interrelated, but they are not, strictly speaking, the same question. It is central to our understanding of the things around us that they continue to exist when we aren't perceiving them, that they be external to us rather than in our minds, and that they be independent of us rather than dependent on us. Shepherd's answer to the first question is particularly interesting. She argues that the human mind naturally (but rationally) infers that objects must continue to exist when we are not perceiving them because that is what is required to fit our experience:
For the mind perceives that unless they are created purposely, ready to appear, upon each irregular call of the senses, they must CONTINUE to exist, ready to appear to them upon such calls.(EPEW, pp. 13-14)
We all know the phenomenon she is noting here: e.g., I look at the computer screen, look away and touch it, look back to it, touch it again, look away and touch it again, &c. If the computer continues to exist unperceived, this explains very well why it is that I can rely on it to be there at each "irregular call of the senses". It's not a mere pattern in my sensation (because it is irregular).
Strictly speaking, this doesn't tell us anything about the nature of the continued existence of the computer. Because it has a 'readiness to appear' that must be accounted for by an adequate cause, I know through my irregular calls of the senses that the cause of its appearing must be something that continues to exist when it isn't appearing, because it is only such a cause that can be ready when I am not looking at it to appear when I am. But this simple reasoning doesn't import much about the nature of the cause. And indeed, Shepherd is willing to allow that there's a lot we don't know about the cause. On her view, the sensory impressions we actually have - colors, shapes, sounds, etc. - are not the objects themselves but signs of the objects that we use quasi-algebraically to stand for the unknown causes of which they are the marks. Shepherd's account of causation does allow us to build up, slowly, over time, some notion of what these causes really are. But it does allow us to say that for every sign of the object we have from the senses there must be something corresponding to it in the object:
I repeat it, therefore, that the unknown causes of all our perceptions, are as the unknown quantities in algebra, which yet may be measured, valued, reasoned on by their signs; and the signs of these outward objects are the sensations they can create; and they may always be spoken of, and compared together, as though they did truly exist, in these forms in which they appear to the mind.
(EPEW pp. 47-48)
Her answer to the second question is also interesting (and closely related), since she argues that it has to do with the fact that all our sensations involve the being-affected of sensory organs, and that this causality is what distinguishes 'external' from 'internal'. Similar considerations show the origin of our notion of independent existence. (One of the strengths of Shepherd's account is that it explains why, despite the fact that they are not the same question, we tend to answer the above three questions in much the same way.)
As I've said before, Shepherd is perhaps the most underappreciated philosopher of the early modern period (or nineteenth century, depending on how you choose to divide the era and where you decide to place her: her dates are 1777-1847).