Hence it is proper to make the strictest examination into the ideas of this sense, and their properties, since it is probable, from the analogies every where conspicuous in natural things, that these are patterns of all the rest. Their peculiar vividness and precision may therefore be considered as serving like a microscope in respect of other ideas, i.e. as magnifying their properties.
Secondly, The vividness and precision here spoken of relate chiefly to distance, magnitude, motion, figure, and position, i.e. to the things considered in the fifty-eighth proposition. However, colours leave distinct ideas of themselves; but then they require an exertion of our voluntary powers for the most part, whereas the ideas of distances, magnitudes, &c. recur incessantly in the trains which pass over the fancy.
Thirdly, The peculiar vividness and precision of visible ideas may probably be owing to the following causes, as well as to some peculiar unknown structure of the optic nerve, and corresponding region of the brain; viz. the perpetual recurrency of visible objects, either the same or similar ones, during the whole time that we are awake; the distinct manner in which they are impressed by means of the several proper conformations of the eye; and their being received in general upon the same part of the retina, precisely or nearly. For, when we view any object with attention, we make the central point of it fall upon the central part of the retina. Farther, as the optic nerve sends off no branches, but is spent wholly upon the retina, this may perhaps contribute in some degree. And these considerations may a little help us to conceive, how the optic nerve, and corresponding region of the brain, may be the repository of such an immense variety of visible ideas, as they are in fact.
Fourthly, The idea of every familiar object has, for the most part, some particular magnitude, position, and aggregate of associates, in its recurrences to the mind. And this somewhat lessens the difficulty mentioned in the last paragraph. The reason of this fourth observation is, that though every visible object appears under different magnitudes, in different positions, and with different associates, yet these differences destroy one another, so that the strongest particularity only remains. However, changes are made from time to time, each subsisting for a short period, and then giving way to the next in succession.
Fifthly, We have fictitious visible ideas of places and persons that we have never seen, as well as of those which we have. These are derived from association evidently, and they often undergo successive changes, like those spoken of in the last paragraph.
Sixthly, Our visible ideas are subject to the voluntary power in a high degree, and may be called up by the slightest associated circumstance, at the same time that they have very numerous connexions with other ideas, and with actual im-
