Thomas Jefferson Papers

William McIlhenney to Thomas Jefferson, 6 April 1816

From William McIlhenney

Philada April 6. 1816


I take the liberty of submitting to your consideration some remarks on perception, which altho’ they throw no light on that mysterious faculty may tend to shew the errors which arise from the use of indefinite terms. I particularly1 allude to the use of the word image in explaining vision.

I shall remark that our knowledge of each of the senses is alike limited, and when rigidly examined will be found to consist of but a few facts derived from sensation and experience. The first conclusion formed by the human mind appears to be that every change must proceed from some cause. whenever we perceive a change we are excited to the discovery of the cause (as each new sensation must necessarily interest a sensitive being) and to the cause when discovered (or beleived so to be) we give a name; and in common language we say2 we perceive the object, when in fact we are only conscious of a new3 perception or change, which we from reason or experience conclude is occasioned by a certain object. Thus when absolute stillness is interrupted, we say we hear something; more correctly we have a perception of the ear which we conclude must proceed from some cause—if upon examination we can find that this effect is connected with a tremulous4 body of a certain form, we give that body a name, and then say I hear a Bell—when in fact we are only conscious of a perception—

When a change is produced in the appearance of the æther or surrounding medium, we have a new sensation which we call colour, accompanied by outline, we seek in like manner for the cause, and discovering reflecting bodies by touch5 we distinguish them by names and then say, I see a man, or a tree, or something; when in fact we have only a perception of change by the eye, which we have learned to proceed from rays of light reflected by surfaces capable of reflection—

When we attempt to move the hand and experience resistance, we have a new sensation and discovering the resisting body give it a name, and then say I feel a ball or spoon, when in fact we have only a sensation which we have learned proceeds from a body of a certain form. The same in taste.

Mr Hume understanding the expression “I see a table” literally has shewn its absurdity by proving the change of apparent magnitude, whereas if he had considered it as used figuratively (the cause for the effect) he would have perceived that by the laws of light those variations of apparent magnitude were the necessary result of reflection from the same real magnitude in different angles. He has therefore substituted the word image or picture as the cause of perception, and endeavours to weaken the belief of real existences, and introduce Phantoms in their place. I shall now shew that the words image and picture are without meaning, and like the Nymph Echo, have but a nominal existence—we shall be obliged therefore to return to the old theory of external objects being the causes of our perceptions, and, without explaining6 the modus operandi, be content to say the Author of nature has so willed it.

“It is a law of our nature, says Dr Ried, established by the will of the Supreme Being, that we perceive no external object, but by means of the organs given us for that purpose. We know how the eye forms a picture of the visible object on the retina; but how this picture makes us see the object we know not—” “we know that in vision an image of the visible object is formed in the bottom of the eye by the rays of light.” It is unnecessary for me to make further quotations from this author to shew that by image or picture he understood something different from the object and the perception—many other authors have expressly stated this as their conception, for they have enquired how it is that we see an object single when an image is formed in each eye, or erect7 when the images are inverted. When they thus speak of two images and inverted, while they admit that the object is both really and by perception single and erect, they must of necessity speak of some thing intervening between the object and perception; and this they call an image or picture.

Is it true, as asserted by these authors, that in vision an image of the visible object is (in any sense) formed in the bottom of the eye. We are not conscious of the existence of such an object. But it may be said it has been proved by the following experiment. “Take a bullock’s eye while it is fresh, and having cut off the three coats from the back part, quite to the vitreous humour put a piece of white paper over that part and hold it to any coloured object, you will see an inverted picture of the object upon the paper8—Is there in fact any thing upon the paper?—does the paper do any more than receive and transmit rays of light from the coloured object, and these rays being converged and crossed before passing to the eye, must they not of necessity, from the laws of vision, change the apparent position of the body? When we look at an object thro’ a convex glass the apparent magnitude9 is changed, yet we say we see the object, and not a picture of it. But when we see an object in a reflecting mirror we then fancy we see a picture, altho nothing exists on the surface of the mirror, and the only difference10 is that by reflecting the rays from the opposed body, the apparent position is changed.

The question why impressions on two eyes, or two ears, or many nerves of sensation, produce single perceptions, may be answered when we discover how our perceptions and sensations are connected, and the mode of communication between mind and body— This will I believe allways remain among the arcana of nature—Philosophers having fancied they had discovered the cause of complete perception in each eye, thought it necessary to account for the single effect resulting from two causes—Take away the doctrine of images altogether, and you save the time and ingenuity wasted on a frivolous question.

It will follow from this analysis that we do not, strictly11 speaking, perceive external objects by any of the senses, and that their existence is a conclusion of the understanding from sensations—It will of course remain a question of innocent speculation whether Berkly’s theory be true or false—It is not difficult to determine whether his Guess has more of probability than the common sense of mankind.

I shall be much indebted to you for your opinion on this subject; and if you should find any thing new, I trust in your hands it may become of importance. you will not be surprised that the word image has been so long suffered to impose on the world, when the word idea has from the time of Aristotle until Dr Ried been used to express a real existence—

I am respectfully Your obt sert

Wm McIlhenney Jr

It is my particular request that whether you consider this letter worthy of notice or not, you would do me the favour of destroying it immediately on perusing it. Were I not an entire stranger to you and sure of remaining so, I should not [have]12 the confidence to submit my Crude thoughts to your notice.

RC (MHi); dateline between signature and postscript; addressed: “Thomas Jefferson Esquire Montecello Virginia”; franked; postmarked Philadelphia, 6 Apr.; endorsed by TJ as received 3 May 1816 and so recorded in SJL.

William McIlhenney (ca. 1779–1854), attorney and diplomat, received an A.B. from the University of Pennsylvania in 1797. In 1820 he became the librarian of the Athenæum of Philadelphia, and in 1823 he was chosen as its secretary, holding both positions for the rest of his life. For at least a decade starting in 1839, McIlhenney was consul for Venezuela at Philadelphia (University of Pennsylvania: Biographical Catalogue of the Matriculates of the College, 1749–1893 [1894], 39; The Charter, By-Laws, and Seventy-Fifth Annual Report of the Athenæum of Philadelphia, to which is prefixed A Sketch of its History [1890], 61; A. M’Elroy’s Philadelphia Directory for 1839 [1839], 324; McElroy’s Philadelphia Directory for 1849 [1849], 238).

David hume explored ideas of space, time, and perception in the section “of the understanding” that constituted the first volume of his work, A Treatise of Human Nature: being An Attempt to introduce the experimental Method of Reasoning into Moral Subjects, 3 vols. (London, 1739).The nymph echo was said to have refused the attentions of the god Pan, who caused her death by sending mad shepherds to attack her. The earth absorbed her body, but her voice lingered (OCD description begins Simon Hornblower and Antony Spawforth, eds., The Oxford Classical Dictionary, 2003 description ends , 502).McIlhenney quotes from Thomas Reid (ried), Essays on the Intellectual Powers of Man (Edinburgh, 1785), 103, 104.The experiment using a bullock’s eye appeared in numerous scientific works, including James Ferguson, Lectures on Select Subjects in Mechanics, Hydrostatics, Pneumatics, and Optics (London, 1760; Sowerby, description begins E. Millicent Sowerby, comp., Catalogue of the Library of Thomas Jefferson, 1952–59, 5 vols. description ends no. 3735), 223.George Berkeley’s (berkly’s) work, An Essay Towards a New Theory of Vision (Dublin, 1709), examined the psychology of perception.

1Manuscript: “particulary.”

2Preceding two words interlined.

3Word interlined.

4Word interlined.

5Preceding two words interlined.

6Word interlined.

7Manuscript: “errect.”

8Omitted closing quotation mark editorially supplied.

9Manuscript: “magnitudes.”

10Word interlined in place of “effect.”

11Manuscript: “strickly.”

12Omitted word editorially supplied.

Index Entries

  • An Essay Towards a New Theory of Vision (G. Berkeley) search
  • Aristotle; referenced search
  • A Treatise of Human Nature (D. Hume) search
  • Berkeley, George; An Essay Towards a New Theory of Vision search
  • cattle; bulls’ eyes used in optical experiments search
  • Echo (mythological character) search
  • Essays on the Intellectual Powers of Man (T. Reid) search
  • Ferguson, James; Lectures on Select Subjects in Mechanics, Hydrostatics, Pneumatics, and Optics search
  • Hume, David; A Treatise of Human Nature search
  • Lectures on Select Subjects in Mechanics, Hydrostatics, Pneumatics, and Optics (J. Ferguson) search
  • McIlhenney, William (of Philadelphia); identified search
  • McIlhenney, William (of Philadelphia); letter from search
  • McIlhenney, William (of Philadelphia); on sensory perception search
  • psychology, experimental; and human perception search
  • Reid, Thomas; Essays on the Intellectual Powers of Man search
  • science; experimental psychology search