Notwithstanding I have told you my design of considering minutely the variety of lines, which serve to raise the ideas of bodies in the mind, and which are undoubtedly to be consider'd as drawn on the surfaces only of solid or opake bodies: yet the endeavouring to conceive, as accurate an idea as is possible, of the _inside_ of those surfaces, if I may be allow'd the expression, will be a great assistance to us in the pursuance of our present enquiry. In order to my being well understood, let every object under our consideration, be imagined to have its inward contents scoop'd out so nicely, as to have nothing of it left but a thin shell, exactly corresponding both in its inner and outer surface, to the shape of the object itself: and let us likewise suppose this thin shell to be made up of very fine threads, closely connected together, and equally perceptible, whether the eye is supposed to observe them from without, or within; and we shall find the ideas of the two surfaces of this shell will naturally coincide. The very word, shell, makes us seem to see both surfaces alike. The use of this conceit, as it may be call'd by some, will be seen to be very great, in the process of this work: and the oftner we think of objects in this shell-like manner, we shall facilitate and strengthen our conception of any particular part of the surface of an object we are viewing, by acquiring thereby a more perfect knowledge of the whole, to which it belongs: because the imagination will naturally enter into the vacant space within this shell, and there at once, as from a center, view the whole form within, and mark the opposite corresponding parts so strongly, as to retain the idea of the whole, and make us masters of the meaning of every view of the object, as we walk round it, and view it from without. Thus the most perfect idea we can possibly acquire of a sphere, is by conceiving an infinite number of straight rays of equal lengths, issuing from the center, as from the eye, spreading every way alike; and circumscribed or wound about at their other extremities with close connected circular threads, or lines, forming a true spherical shell. But in the common way of taking the view of any opake object, that part of its surface, which fronts the eye, is apt to occupy the mind alone, and the opposite, nay even every other part of it whatever, is left unthought of at that time: and the least motion we make to reconnoitre any other side of the object, confounds our first idea, for want of the connexion of the two ideas, which the complete knowledge of the whole would naturally have given us, if we had considered it in the other way before.