131.  Thirdly, it is, I think, an axiom universally received that quantities of the same kind may be added together and make one entire sum.  Mathematicians add lines together:  but they do not add a line to a solid, or conceive it as making one sum with a surface:  these three kinds of quantity being thought incapable of any such mutual addition, and consequently of being compared together in the several ways of proportion, are by then esteemed entirely disparate and heterogeneous.  Now let anyone try in his thoughts to add a visible line or surface to a tangible line or surface, so as to conceive them making one continued sum or whole.  He that can do this may think them homogeneous:  but he that cannot, must by the foregoing axiom think them heterogeneous.  A blue and a red line I can conceive added together into one sum and making one continued line:  but to make in my thoughts one continued line of a visible and tangible line added together is, I find, a task far more difficult, and even insurmountable:  and I leave it to the reflexion and experience of every particular person to determine for himself.

132.  A farther confirmation of our tenet may be drawn from the solution of Mr. Molyneux’s problem, published by Mr. Locke in his essay:  which I shall set down as it there lies, together with Mr. Locke’s opinion of it, ’"Suppose a man born blind, and now adult, and taught by his touch to distinguish between a cube and a sphere of the same metal, and nighly [Sic] of the same bigness, so as to tell, when he felt one and t’other, which is the cube and which the sphere.  Suppose then the cube and sphere placed on a table, and the blind man to be made to see:  Quaere, whether by his sight, before he touched them, he could now distinguish and tell which is the globe, which the cube?” To which the acute and judicious proposer answers:  “Not.  For though he has obtained the experience of how a globe, how a cube, affects his touch, yet he has not yet attained the experience that what affects his touch so or so must affect his sight so or so:  or that a protuberant angle in the cube that pressed his hand unequally shall appear to his eye as it doth in the cube.”  I agree with this thinking gentleman, whom I am proud to call my friend, in his answer to this his problem; and am of opinion that the blind man at first sight would not be able with certainty to say which was the globe, which the cube, whilst he only saw them.’ (Essay on human understanding, B. ii.  C. 9.  S. 8.)

133.  Now, if a square surface perceived by touch be of the same sort with a square surface perceived by sight, it is certain the blind man here mentioned might know a square surface as soon as he saw it:  it is no more but introducing into his mind by a new inlet an idea he has been already well acquainted with.  Since, therefore, he is supposed to have known by his touch that a cube is a body terminated