to represent the sound annexed to it, in the formation whereof there being eight different collisions or modifications of the air by the organs of speech, each of which produces a difference of sound, it was fit the word representing it should consist of as many distinct characters, thereby to mark each particular difference or part of the whole sound.  And yet nobody, I presume, will say the single letter a, or the word adultery, are like unto, or of the same species with, the respective sounds by them represented.  It is indeed arbitrary that, in general, letters of any language represent sounds at all:  but when that is once agreed, it is not arbitrary what combination of letters shall represent this or that particular sound.  I leave this with the reader to pursue, and apply it in his own thoughts.

144.  It must be confessed that we are not so apt to confound other signs with the things signified, or to think them of the same species, as we are visible and tangible ideas.  But a little consideration will show us how this may be without our supposing them of a like nature.  These signs are constant and universal, their connexion with tangible ideas has been learnt at our first entrance into the world; and ever since, almost every moment of our lives, it has been occurring to our thoughts, and fastening and striking deeper on our minds.  When we observe that signs are variable, and of human institution; when we remember there was a time they were not connected in our minds with those things they now so readily suggest; but that their signification was learned by the slow steps of experience:  this preserves us from confounding them.  But when we find the same signs suggest the same things all over the world; when we know they are not of human institution, and cannot remember that we ever learned their signification, but think that at first sight they would have suggested to us the same things they do now:  all this persuades us they are of the same species as the things respectively represented by them, and that it is by a natural resemblance they suggest them to our minds.

145.  Add to this that whenever we make a nice survey of any object, successively directing the optic axis to each point thereof, there are certain lines and figures described by the motion of the head or eye, which being in truth perceived by feeling, do nevertheless so mix themselves, as it were, with the ideas of sight, that we can scarce think but they appertain to that sense.  Again, the ideas of sight enter into the mind several at once, more distinct and unmingled than is usual in the other senses beside the touch.  Sounds, for example, perceived at the same instant, are apt to coalesce, if I may so say, into one sound:  but we can perceive at the same time great variety of visible objects, very separate and distinct from each other.  Now tangible extension being made up of several distinct coexistent parts, we may hence gather another reason that may dispose