Its occasion and design, and its connexion with his spiritual conception of the material world, are suggested in Sections 43 and 44 of the _Principles_. Those sections are a key to the _Essay_. They inform us that in the _Essay_ the author intentionally uses language which seems to attribute a reality independent of all percipient spirit to the ideas or phenomena presented in Touch; it being beside his purpose, he says, to “examine and refute” that “vulgar error” in “a work on Vision.” This studied reticence of a verbally paradoxical conception of Matter, in reasonings about vision which are fully intelligible only under that conception, is one cause of a want of philosophical lucidity in the _Essay_. Another circumstance adds to the embarrassment of those who approach the _Principles_ and the three _Dialogues_ through the _Essay on Vision_. The _Essay_ offers no exception to the lax employment of equivocal words familiar in the early literature of English philosophy, but which is particularly inconvenient in the subtle discussions to which we are here introduced. At the present day we are perhaps accustomed to more precision and uniformity in the philosophical use of language; at any rate we connect other meanings than those here intended with some of the leading words. It is enough to refer to such terms as _idea_, _notion_, _sensation_, _perception_, _touch_, _externality_, _distance_, and their conjugates. It is difficult for the modern reader to revive and remember the meanings which Berkeley intends by _idea_ and _notion_—so significant in his vocabulary; and _touch_ with him connotes muscular and locomotive experience as well as the pure sense of contact. Interchange of the terms _outward_, _outness_, _externality_, _without the mind_, and _without the eye_ is confusing, if we forget that Berkeley implies that percipient mind is virtually coextensive with our bodily organism, so that being “without” or “at a distance from” our bodies is being at a distance from the percipient mind. I have tried in the annotations to relieve some of these ambiguities, of which Berkeley himself warns us (cf. sect. 120). The _Essay_ moreover abounds in repetitions, and interpolations of antiquated optics and physiology, so that its logical structure and even its supreme generalisation are not easily apprehended. I will try to disentangle them. ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐