What was Woolf thinking of when she wrote “wildest fancies,” I ask myself; what had she read that I haven't, which passage was going through her mind? “[M]any scholars and critics today would agree with Woolf's view that Cavendish lacked discipline,” wrote James Fitzmaurice. C.H. Firth referred to her “facile pen.” When he wanted to criticise her plays he complained that “her characters are mere abstractions, qualities, and humours, uttering the fantastic speeches and quaint conceits which she loved to write.”
Firth's opinions might have affected Woolf's, seeing that he edited at least one of the books that she is reviewing in her essay (The Life of William Cavendish, Duke of Newcastle, Etc). In fact, stopping and considering, I think his introduction has the answer to my question. Woolf might not have been thinking about a piece of Cavendish. She might have been thinking of this, written by Firth.
… the ponderous tomes on science and philosophy which the Duchess published are entirely valueless. This was not only due to the ignorance of the writings of others, which the Duchess admits, but to the method which she adopted in reasoning on physical science. One of her correspondents, Glanville, points this out to her. "There are two sorts of reasoning," he says, "those that the mind advanceth from its inbred store, such are all metaphysical contemplations; and those natural researches which are raised from experiment and the objects of sense. Now, what I have said about these matters is to tie down the mind in physical things to consider nature as it is, to lay a foundation in sensible collections, and from thence to proceed to general propositions and discourses. So that my aim is that we may arise according to the order of nature, from the exercise of our senses to that of our reason; which indeed is most noble and most perfect when it concludes aright, but not so when it is mistaken; and that it may so conclude and arrive to that perfection it must begin in sense; and the more experiments our reasons have to work on, by so much they are the more likely to be certain in their conclusions, and consequently more perfect in their actings." Whilst the Royal Society and all those to whom the progress of physical science in England during the latter half of the seventeenth century was due were eagerly pursuing the experimental method, the Duchess continued to spin her "metaphysical cogitations" like a spider, as she says, from her own brain.
Which is another way of saying that she was making “wildest fancies” – the wildness especially noticeable by Firth and Woolf because they knew that she was going against the advice of people who were already established in the areas where she wanted to speculate. She rejected them in favour of her personal metaphysics. "I desire all those that are friends to my book to believe that whatsoever is new is my own, which I hope all is; for I never had any guide to direct me, nor intelligence from any authors to advertise me, but writ according to my own natural cogitations." (Preface to Philosophical Opinions (1655), quoted by Firth.) “Her love of singularity amounted to a passion.” (Firth.) Powys again.