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, [pg 097] 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.
The reader must remember that this Essay on Vision is professedly an introspective appeal to human consciousness. It is an analysis of what human beings are conscious of when they see, the results being here and there applied, partly by way of verification, to solve some famous optical or physiological puzzle. The aim is to present the facts, the whole facts, and nothing but the facts of our internal visual experience, as distinguished from supposed facts and empty abstractions, which an irregular exercise of imagination, or abuse of words, had put in their place. [pg 098] The investigation, moreover, is not concerned with Space in its metaphysical infinity, but with finite sections of Space and their relations, which concern the sciences, physical and mathematical, and with real or tangible Distance, Magnitude, and Place, in their relation to seeing.
From the second section onwards the Essay naturally falls into six Parts, devoted successively to the proof of the six following theses regarding the relation of Sight to finite spaces and to things extended:—
I. (Sect. 2-51.) Distance, or outness from the eye in the line of vision, is not seen: it is only suggested to the mind by visible phenomena and by sensations felt in the eye, all which are somehow its arbitrarily constituted and non-resembling Signs.
II. (Sect. 52-87.) Magnitude, or the amount of space that objects of sense occupy, is really invisible: we only see a greater or less quantity of colour, and colour depends upon percipient mind: our supposed visual perceptions of real magnitude are only our own interpretations of the tactual meaning of the colours we see, and of sensations felt in the eye, which are its Signs.
III. (Sect. 88-120.) Situation of objects of sense, or their real relation to one another in ambient space, is invisible: what we see is variety in the relations of colours to one another: our supposed vision of real tangible locality is only our interpretation of its visual non-resembling Signs.
IV. (Sect. 121-46.) There is no object that is presented in common to Sight and Touch: space or extension, which has the best claim to be their common object, is specifically as well as numerically different in Sight and in Touch.