Let attention also be called to one great source of error, which is also a great source of power, namely, that much of our thinking is carried on by signs instead of images.  We use words as signs of objects; these suffice to carry on the train of inference, when very few images of the objects are called up.  Let any one attend to his thoughts and he will be surprised to find how rare and indistinct in general are the images of objects which arise before his mind.  If he says “I shall take a cab and get to the railway by the shortest cut,” it is ten to one that he forms no image of cab or railway, and but a very vague image of the streets through which the shortest cut will lead.  Imaginative minds see images where ordinary minds see nothing but signs:  this is a source of power; but it is also a source of weakness; for in the practical affairs of life, and in the theoretical investigations of philosophy, a too active imagination is apt to distract the attention and scatter the energies of the mind.

In complex trains of thought signs are indispensable.  The images, when called up, are only vanishing suggestions:  they disappear before they are more than half formed.  And yet it is because signs are thus substituted for images (paper transacting the business of money) that we are so easily imposed upon by verbal fallacies and meaningless phrases.  A scientific man of some eminence was once taken in by a wag, who gravely asked him whether he had read Bunsen’s paper on the malleability of light.  He confessed that he had not read it:  “Bunsen sent it to me, but I’ve not had time to look into it.”

The degree in which each mind habitually substitutes signs for images will be, ceteris PARIBUS, the degree in which it is liable to error.  This is not contradicted by the fact that mathematical, astronomical, and physical reasonings may, when complex, be carried on more suecessfully by the employment of signs; because in these cases the signs themselves accurately represent the abstractness of the relations.  Such sciences deal only with relations, and not with objects; hence greater simplification ensures greater accuracy.  But no sooner do we quit this sphere of abstractions to enter that of concrete things, than the use of symbols becomes a source of weakness.  Vigorous and effective minds habitually deal with concrete images.  This is notably the case with poets and great literates.  Their vision is keener than that of other men.  However rapid and remote their flight of thought, it is a succession of images, not of abstractions.  The details which give significance, and which by us are seen vaguely as through a vanishing mist, are by them seen in sharp outlines.  The image which to us is a mere suggestion, is to them almost as vivid as the object.  And it is because they see vividly that they can paint effectively.