It is in vain to say that a habit of association of ideas from observing "contiguity in time, and place," between objects is all we know of power; a habit of the mind will not begin existence, will not introduce a quality. The really philosophical method of viewing the subject is this: that objects in relation to us, are nothing but masses of certain qualities, affecting certain of our senses; and which, when independent of our senses, are unknown powers or qualities in nature. These masses change their qualities by their mixture with any other mass, and then the corresponding qualities determined to the senses must of course also change. These changed qualities, are termed effects; or consequents; but are really no more than NEW QUALITIES arising from new objects, which have been formed by the junctions of other objects (previously formed) or might be considered as the unobserved qualities of existing objects; which shall be observed when properly exhibited.
If then an existence now in being, conjoined with any other, forms thereby a new nature, capable of exhibiting new qualities, these new qualities must enter into the definition of the objects; they become a part of their natures; and when by careful experiment, or judicious observation, no new prevening circumstances are supposed to make an alteration in the conjunction of the same bodies, the new qualities, that are named effects, are expected without a doubt to arise upon every such conjunction; because , they as much belong to this newly combined nature, as the original qualities did to each separate nature, before their conjunction.
This lovely little passage, in which Shepherd is attacking Hume's claim that our notions of causes and effects are due to custom or reasoning, summarizes a number of features of Shepherd's theory of causation.
(1) Objects can be treated as masses of 'qualities' or characteristics.
(2) The mixtures of these masses of qualities is causation.
(3) The effects are the objects that are masses of the mixed qualities.
(See here for a very rough, crude summary of what's involved in these claims.)
Further (although these are only seen clearly when this passage is looked at in its full context):
(4) Because of (1), (2), and (3) we can reason cogently that similar qualities in similar circumstances will have similar effects (i.e., we can provide a rational, and not merely customary basis, for the maxim that like causes have like effects), and thus of induction insofar as it involves this maxim.
(5) Using (1), (2), and (3) we can provide a rational basis for the maxim that everything that begins to exist must have a cause.
Of course, these are not the only problems that arise for a theory of causation, and Shepherd discusses a number of others. But I thought that this passage provides a neat little summary of some of her views.
Be prepared for lots more Shepherd in the coming weeks.