This isn't really meant to stand alone, but I've been meaning for a while to post something on Lady Mary Shepherd's account of how we know the external world as well as Whewell's account of science. Both of those I'll probably have time to do in the next few days, but for the former, since Shepherd's account depends on her prior account of causation, I wanted to post a brief post laying out, in rough and summary form, her primary argument for a key claim, so I can refer back to it in the post to come.
To be proved: Whatever begins to exist has a cause.
1. Beginning to exist is the introduction of a difference. (premise)
2. The introduction of a difference is an action. (premise)
3. An action is an exercised capability that belongs to the nature of an existing thing. (premise)
4. An existing thing cannot be a thing that does not yet exist. (premise)
5. What has a beginning to its existence did not yet exist before a difference was introduced. (premise)
6. The beginning to exist of what begins to exist is not the action of what has a beginning to its existence, but rather the action of an existing thing distinct from it. (from (1)-(5) combined)
7. Therefore everything that has a beginning to its existence does so as the action of something that exists and is not itself. (from (6) directly)
And (7) is equivalent to what was to be proved. Shepherd regards (1)-(5) as all necessary truths; anyone who denies them simply doesn't understand what an action is, or what we mean when we say something begins to exist.
In actuality, different routes are possible, and we find such different routes in Shepherd's discussions. But they are all similar in recognizing that nothing begins to exist unless a difference is introduced, that the introduction of a difference is an action, and that actual actions cannot be the actions of something that is itself not actual.
It's this line of thought that leads Shepherd to reject unequivocally the claim that cause and effect is simply a regularity or invariable succession, and especially to reject Hume's claim that we infer causes wholly on the basis of custom and not reasoning.