parts of several different and inconsistent ideas are put together’ essay on Hum.  Understand.  B. iv.  C. 7.  S.9.  This is the idea which he thinks needful for the enlargement of knowledge, which is the subject of mathematical demonstration, and without which we could never come to know any general proposition concerning triangles.  That author acknowledges it doth ’require some pains and skill to form this general idea of a triangle.’  Ibid.  But had he called to mind what he says in another place, to wit, ’That ideas of mixed modes wherein any inconsistent ideas are put together cannot so much as exist in the mind, i.e. be conceived.’  VID.  B. iii.  C. 10.  S. 33.  Ibid.  I say, had this occurred to his thoughts, it is not improbable he would have owned it above all the pains and skill he was master of to form the above-mentioned idea of a triangle, which is made up of manifest, staring contradictions.  That a man who laid so great a stress on clear and determinate ideas should nevertheless talk at this rate seems very surprising.  But the wonder will lessen if it be considered that the source whence this opinion flows is the prolific womb which has brought forth innumerable errors and difficulties in all parts of philosophy and in all the sciences:  but this matter, taken in its full extent, were a subject too comprehensive to be insisted on in this place.  And so much for extension in abstract.

126.  Some, perhaps, may think pure space, vacuum, or trine dimension to be equally the object of sight and touch:  but though we have a very great propension to think the ideas of outness and space to be the immediate object of sight, yet, if I mistake not, in the foregoing parts of this essay that hath been clearly demonstated to be a mere delusion, arising from the quick and sudden suggestion of fancy, which so closely connects the idea of distance with those of sight, that we are apt to think it is itself a proper and immediate object of that sense till reason corrects the mistake.

127.  It having been shown that there are no abstract ideas of figure, and that it is impossible for us by any precision of thought to frame an idea of extension separate from all other visible and tangible qualities which shall be common both to sight and touch:  the question now remaining is, whether the particular extensions, figures, and motions perceived by sight be of the same kind with the particular extensions, figures, and motions perceived by touch?  In answer to which I shall venture to lay down the following proposition:  The extension, figures, and motions perceived by sight are specifically distinct from the ideas of touch called by the same names, nor is there any such thing as one idea or kind of idea common to both senses.  This proposition may without much difficulty be collected from what hath been said in several places of this essay.  But because it seems so remote from, and contrary to, the received notions and settled opinion of mankind, I shall attempt to demonstrate it more particularly and at large by the following arguments.