1. FROM The Problems of Philosophy [1]

In daily life, we assume as certain many things which, on a closer scrutiny, are found to be so full of apparent contradictions that only a great amount of thought enables us to know what it is that we really may believe. In the search for certainty, it is natural to begin with our present experiences, and in some sense, no doubt, knowledge is to be derived from them. But any statement as to what it is that our immediate experiences make us know is very likely to be wrong. It seems to me that I am now sitting in a chair, at a table of a certain shape, on which I see sheets of paper with writing or print. By turning my head I see out of the window buildings and clouds and the sun. I believe that the sun is about ninety-three million miles from the earth; that it is a hot globe many times bigger than the earth; that, owing to the earth's rotation, it rises every morning, and will continue to do so for an indefinite time in the future. I believe that, if any other normal person comes into my room, he will see the same chairs and tables and books and papers as I see, and that the table which I see is the same as the table which I feel pressing against my arm. All this seems to be so evident as to be hardly worth stating, except in answer to a man who doubts whether I know anything. Yet all this may be reasonably doubted, and all of it requires much careful discussion before we can be sure that we have stated it in a form that is wholly true.

To make our difficulties plain, let us concentrate attention on the table. To the eye it is oblong, brown and shiny, to the touch it is smooth and cool and hard; when I tap it, it gives out a wooden sound. Anyone else who sees and feels and hears the table will agree with this description , so that it might seem as if no difficulty would arise; but as soon as we try to be more precise our troubles begin. Although I believe that the table is "really" of the same colour all over, the parts that reflect the light look much brighter than the other parts, that, if I move, the parts that reflect the light will be different, so that the apparent distribution of colours on the table will change. It follows that if several people are looking at the table at the same moment, no two of them will see exactly the same distribution of colours, because no two can see it from exactly the same point of view, and any change in the point of view makes some change in the way the light is reflected.

For most practical purposes the differences are unimportant, but to the painter they are all-important: the painter has to unlearn the habit of thinking that things seem to have the colour which common sense says they "really" have, and to learn the habit of seeing things as they appear. Here we have already the beginning of one of the distinctions that cause most trouble in philosophy-the distinction between "appearance" and "reality:' between what things seem to be and what they are. The painter wants to know what things seem to be, the practical man and the philosopher want to know what they are; but the philosopher's wish to know this is stronger than the practical man's, and is more troubled by knowledge as to the difficulties of answering the question.

To return to the table. It is evident from what we have found, that there is no colour which preeminently appears to be the colour of the table, or even of anyone particular part of the table--it appears to be of different colours from different points of view, and there is no reason for regarding some of these as more really its colour than others. And we know that even from a given point of view the colour will seem different by artificial light, or to a colour-blind man, or to a man wearing blue spectacles, while in the dark there will be no colour at all, though to touch and hearing the table will be unchanged. This colour is not something which is inherent in the table, but something depending upon the table and the spectator and the way the light falls on the table. When, in ordinary life, we speak of the colour of the table, we only mean the sort of colour which it will seem to have to a normal spectator from an ordinary point of view under usual conditions of light. But the other colours which appear under other conditions have just as good a right to be considered real; and therefore, to avoid favouritism, we are compelled to deny that, in itself, the table has anyone particular colour.

The same thing applies to the texture. With the naked eye one can see the grain, but otherwise the table looks smooth and even. If we looked at it through a microscope, we should see roughnesses and hills and valleys, and all sorts of differences that are imperceptible to the naked eye. Which of these is the "real" table? We are naturally tempted to say that what we see through the microscope is more real, but that in turn would be changed by a still more powerful microscope. If, then, we cannot trust what we see with the naked eye, why should we trust what we see through a microscope? Thus, again, the confidence in our sense with which we began deserts us.

The shape of the table is not better. We are all in the habit of judging as to the "real" shapes of things, and we do this so unreflectingly that we come to think we actually see the real shapes. But, in fact, as we all have to learn if we try to draw, a given thing looks different in shape from every different point of view. If our table is "really" rectangular, it will look, from almost all points of view, as if it had two acute angles and two obtuse angles. If opposite sides are parallel, they will look as if they converged to a point away from the spectator; if they are of equal

a table, because experience has taught us to construct the "real" shape from the apparent shape, and the "real" shape is what interests us as practical men. But the "real" shape is not what we see; it is something inferred from what we see. And what we see is constantly changing in shape as we move about the room; so that here again the senses seem not to give us the truth about the table itself, but only about the appearance of the table.

Similar difficulties arise when we consider the sense of touch. It is true that the table always gives us a sensation of hardness, and we feel that it resists pressure. But the sensation we obtain depends upon how hard we press the table and also upon what part of the body we press with; thus the various sensations due to various pressures or various parts of the body cannot be supposed to reveal directly any definite property of the table, but at most to be signs of some property, which causes all the sensations, but is not actually apparent in any of them. And the same applies still more obviously to the sounds which can be elicited by rapping the table.

Thus it becomes evident that the real table, if there is one, is not the same as what we immediately experience by sight or touch or hearing. The real table, if there is one, is not immediately known to us at all, but must be an inference from what is immediately known. Hence, two very difficult questions at once arise; namely, (1) Is there a real table at all? (2) If so, what sort of object can it be?

1. FROM Theatetus[2] BY Plato

SOCRATES: But the question you were asked, Theatetus, was not, what are the objects of knowledge, nor yet how many sorts of knowledge there are. We did not want to count them, but to find out what the thing itself-knowledge-is... . Perception, you say, is knowledge?

THEATETUS: Yes.

SOCRATES: The account you give of the nature of knowledge is not, by any means, to be despised. It is the same that was given by Protagoras, though he stated it in a somewhat different way. He says, you will remember, that "man is the measure of all things-alike of the being of things that are and of the not-being of things that are not." No doubt you have read that.

THEATETUS: Yes, often.

SOCRATES: He puts it in this sort of way, doesn't he, that any given thing "is to me

such as it appears to me and is to you such as it appears to you," you and I being men?

THEATETUS: Yes, that is how he puts it.

SOCRATES: Well, what a wise man says is not likely to be nonsense. So let us follow

his meaning. Sometimes, when the same wind is blowing, one of us feels chilly,

the other does not, or one may feel slightly chilly, the other quite cold.

THEATETUS: Certainly.

SOCRATES : Well, in that case are we to say that the wind in itself is cold or not

cold? Or shall we agree with Protagoras that it is cold to the one who feels chilly

and not to the other? ... [I]ndeed the doctrine is a remarkable one. It declares that

nothing is one thing just by itself, nor can you rightly call it by one definite name, nor

even say it is of any definite sort.

In this way, Socrates claims that the empiricist cannot have any knowledge at all. He defends the rationalist's claim earlier in the dialogue:

SOCRATES: You do not suppose a man can understand the name of a thing when

he does not know what the thing is?

THEATETUS : Certainly not.

SOCRATES: Then, if he has no idea of knowledge, "knowledge about shoes" conveys

nothing to him?

THEATETUS: No.

SOCRATES: "Cobblery" in fact, or the name of any other art has no meaning for

anyone who has no conception of knowledge.

2. FROM "Meditation I"[3] BY Rene Descartess

OF THE THINGS WHICH MAY BE BROUGHT WITHIN THE SPHERE OF THE DOUBTFUL

… It is now some years since I detected how many were the false beliefs that I had from my earliest youth admitted as true, and how doubtful was everything I had since constructed on this basis; and from that time I was convinced that I must once and for all seriously undertake to rid myself of all the opinions which I had formerly accepted, and commence to build anew from the foundation, if I wanted to establish any firm and permanent structure in the sciences.

… Now for this object it is not necessary that I should show that all of these are false-I shall perhaps never arrive at this end. But inasmuch as reason already persuades me that I ought no less carefully to withhold my assent from matters which are not entirely certain and indubitable than from those which appear to me manifestly to be false, if I am able to find in each one some reason to doubt, this will suffice to justify my rejecting the whole. And for that end it will not be requisite that I should examine each in particular, which would be an endless undertaking; for owing to the fact that the destruction of the foundations of necessity brings with it the downfall of the rest of the edifice, I shall only in the first place attack those principles upon which all my former opinions rested.

… All that up to the present time I have accepted as most true and certain I have learned either from the senses or through the senses; but it is sometimes proved to me that these senses are deceptive, and it is wiser not to trust entirely to any thing by which we have once been deceived.

But it may be that although the senses sometimes deceive us concerning things which are hardly perceptible, or very far away, there are yet many others to be met with as to which we cannot reasonably have any doubt, although we recognize them by their means. For example, there is the fact that I am here, seated by the fire, attired in a dressing gown, having this paper in my hands and other similar matters. And how could I deny that these hands and this body are mine, were it not perhaps that I compare myself to certain persons, devoid of sense, whose cerebella are so troubled and clouded by the violent vapours of black bile, that they constantly assure us that they think they are kings when they are really quite poor, or that they are clothed in purple when they are really without covering, or who imagine that they have an earthenware head or are nothing but pumpkins or are made of glass. But they are mad, and I should not be any the less insane were I to follow examples so extravagant.

… At the same time I must remember that I am a man, and that consequently I am in the habit of sleeping, and in my dreams representing to myself the same things or sometimes even less probable things, than do those who are insane in their waking moments. How often has it happened to me that in the night I dreamt that I found myself in this particular place, that I was dressed and seated near the fire, whilst in reality I was lying undressed in bed! At this moment it does indeed seem to me that it is with eyes awake that I am looking at this paper; that this head which I move is not asleep, that it is deliberately and of set purpose that I extend my hand and perceive it; what happens in sleep does not appear so clear nor so distinct as does all this. But in thinking over this I remind myself that on many occasions I have in sleep been deceived by similar illusions, and in dwelling carefully on this reflection I see so manifestly that there are no certain indications by which we may clearly distinguish wakefulness from sleep that I am lost in astonishment. And my astonishment is such that it is almost capable of persuading me that I now dream.

…Arithmetic, Geometry and other sciences of that kind which only treat of things that are very simple and very general, without taking great trouble to ascertain whether they are actually existent or not, contain some measure of certainty and an element of the indubitable. For whether I am awake or asleep, two and three together always form five, and the square can never have more than four sides, and it does not seem possible that truths so clear and apparent can be suspected of any falsity [or uncertainty]

…Nevertheless I have long had fixed in my mind the belief that an all-powerful God existed by whom I have been created such as I am. But how do I know that He has not brought it to pass that there is no earth, no heaven, no extended body, no magnitude, no place, and that nevertheless [I possess the perceptions of all these things and that] they seem to me to exist just exactly as 1 now see them? And, besides, as I sometimes imagine that others deceive themselves in the things which they think they know best, how do I know that I am not deceived every time that I add two and three, or count the sides of a square, or judge of things yet simpler, if anything simpler can be imagined? But possibly God has not desired that I should be thus deceived, for He is said to be supremely good. If, however, it is contrary to His goodness to have made me such that I constantly deceive myself, it would also appear to be contrary to His goodness to permit me to be sometimes deceived, and nevertheless I cannot doubt that He does permit this.

…I shall then suppose, not that God who is supremely good and the fountain of truth, but some evil genius not less powerful than deceitful, has employed his whole energies in deceiving me; I shall consider that the heavens, the earth, colours, fIgures, sound, and all other external things are nought but the illusions and dreams of which this genius has availed himself in order to lay traps for my credulity; I shall consider myself as having no hands, no eyes, no flesh, no blood, nor any senses, yet falsely believing myself to possess all these things; I shall remain obstinately attached to this idea, and if by this means it is not in my power to arrive at the knowledge of any truth, I may at least do what is in my power [i .e., suspend my judgment], and with firm purpose avoid giving credence to any false thing, or being imposed upon by this arch deceiver, however powerful and deceptive he may be. But this task is a laborious one, and insensibly a certain lassitude leads me into the course of my ordinary life. And just as a captive who in sleep enjoys an imaginary liberty, when he begins to suspect that this liberty is but a dream, fears to awaken, and conspires with these agreeable illusions that the deception may be prolonged, so insensibly of my own accord I fall back into my former opinions, and I dread awakening from this slumber, lest the laborious wakefulness which would follow the tranquility of this repose should have to be spent not in daylight, but in the excessive darkness of the difficulties which have just been discussed.

3. FROM "Meditation II" BY Rene Descartes[4]

OF THE NATURE OF THE HUMAN MIND;

AND THAT IT IS MORE EASILY KNOWN THAN THE BODY The Meditation of yesterday filled my mind with so many doubts that it is no longer in my power to forget them. And yet I do not see in what manner I can resolve them; and, just as if I had all of a sudden fallen into very deep water, I am so disconcerted that I can neither make certain of setting my feet on the bottom, nor can I swim and so support myself on the surface. I shall nevertheless make an effort and follow anew the same path as that on which I yesterday entered, i.e. I shall proceed by setting aside all that in which the least doubt could be supposed to exist, just as if I had discovered that it was absolutely false; and I shall ever follow in this road until I have met with something which is certain, or at least, if I can do nothing else, until I have learned for certain that there is nothing in the world that is certain. Archimedes, in order that he might draw the terrestrial globe out of its place, and transport it elsewhere, demanded only that one point should be fixed and immovable; in the same way I shall have the right to conceive high hopes if I am happy enough to discover one thing only which is certain and indubitable.

I suppose, then, that all the things that I see are false; I persuade myself that nothing has ever existed of all that my fallacious memory represents to me. I consider that I possess no senses; I imagine that body, figure, extension, movement and place are but the fictions of my mind. What, then, can be esteemed as true? Perhaps nothing at all, unless that there is nothing in the world that is certain.

But how can I know there is not something different from those things that I have just considered, of which one cannot have the slightest doubt? Is there not some God, or some other being by whatever name we call it, who puts these reflections into my mind? That is not necessary, for is it not possible that I am capable of producing them myself? I myself, am I not at least something? But I have already denied that I had senses and body. Yet I hesitate, for what follows from that? Am I so dependent on body and senses that I cannot exist without these? But I was persuaded that there was nothing in all the world, that there was no heaven, no earth, that there were no minds, nor any bodies; was I not then likewise persuaded that I did not exist? Not at all; of a surety I myself did exist since I persuaded myself of something [or merely because I thought of something]. But there is some deceiver or other, very powerful and very cunning, who ever employs his ingenuity in deceiving me. Then without doubt I exist also if he deceives me, and let him deceive me as much as he will, he can never cause me to be nothing so long as I think that I am something. So that after having reflected well and carefully examined all things, we must come to the definite conclusion that this proposition: I am, I exist, is necessarily true each time that I pronounce it, or that I mentally conceive it.

4. FROM "Meditation VI"[5] BY Rene Descartes

Since He has given me a very strong inclination to believe that these ideas (of trees, houses, etc.) arise from corporeal objects, I do not see how he could be vindicated from the charge of deceit, if in truth they proceeded from any other source, or were produced by other causes than corporeal things? (For example, by the evil demon, or in dreams.)

Therefore,

We cannot be deceived [whether by the evil demon or whatever else] . ... I cannot doubt but that there is in me a certain passive faculty of perception, that is, of receiving and taking knowledge of the ideas of sensible things; but this would be useless to me, if there did not also exist in me, or in some other thing, another active faculty capable of forming and producing those ideas. But this active faculty cannot be in me [in as far as I am but a thinking thing], seeing that it does not presuppose thought, and also that those ideas are frequently produced in my mind without my contributing to it in any way, and even frequently contrary to my will. This faculty must therefore exist in some substance different from me; in which all the objective reality of the ideas that are produced by this faculty is contained formally or eminently, as I before remarked: and this substance is either a body, that is to say, a corporeal nature in which is contained formally [and in effect] all that is objectively [and by representation] in those ideas; or it is God himself, or some other creature, of a rank superior to body, in which the same is contained eminently. But as God is no deceiver, it is manifest that he does not of himself and immediately communicate those ideas to me, nor even by the intervention of any creature in which their objective reality is not formally, but only eminently, contained. For as he has given me no faculty whereby I can discover this to be the case, but, on the contrary, a very strong inclination to believe that those ideas arise from corporeal objects, I do not see how he could be vindicated from the charge of deceit, ifin truth they proceeded from any other source, or were produced by other causes than corporeal things: and accordingly it must be concluded, that corporeal objects exist. Nevertheless they are not perhaps exactly such as we perceive by the senses, for their comprehension by the senses is, in many instances, very obscure and confused; but it is at least necessary to admit that all which I clearly and distinctly conceive as in them, that is, generally speaking, all that is comprehended in the object of speculative geometry, really exists external to me.

5. FROM An Essay Concerning Human Understanding[6] ~BY John Locke

1. The way shown how we come by any knowledge, sufficient to prove it not innate. It is an established opinion among some men, that there are in the understanding certain innate principles; some primary notions, characters, as it were, stamped upon the mind of man which the soul receives in its very first being, and brings into the world with it. It would be sufficient to convince unprejudiced readers of the falseness of this supposition, if I should only show (as I hope I shall in the following parts of this discourse) how men, barely by the use of their natural faculties, may attain to all the knowledge they have, without the help of any innate impressions, and may arrive at certainty, without any such original notions or principles . .. .

2. General Assent the great Argument. There is nothing more commonly taken for granted, than that there are certain principles, both speculative and practical (for they speak of both), universally agreed upon by all mankind, which therefore, they argue, must needs be constant impressions which the souls of men receive in their first beings, and which they bring into the world with them, as necessarily and really as they do any of their inherent faculties.

3. Universal Consent proves nothing innate. This argument, drawn from universal consent, has this misfortune in it, that if it were true in matter of fact that there were certain truths wherein all mankind agreed, it would not prove them innate, if there can be any other way shown how men may come to that universal agreement in the things they do consent in, which I presume may be done.

4. "What is, is," and "it is impossible for the same Thing to be and not to be," not universally assented to. But, which is worse, this argument of universal consent, which is made use of to prove innate principles, seems to me a demonstration that there are none such; because there are none to which all mankind give an universal assent. I shall begin with the speculative, and instance in those magnified principles of demonstration, "Whatsoever is, is" and "It is impossible for the same thing to be, and not to be"; which, of all others, I think, have the most allowed title to innate. These have so settled a reputation of maxims universally received that it will no doubt be thought strange if anyone should seem to question it. But yet I take liberty to say that these propositions are so far from having an universal assent that there are a great part of mankind to whom they are not so much as known.

5. Not on the Mind naturally imprinted, because not known to Children, Idiots, &.-For, first, it is evident that all children and idiots have not the least apprehension or thought of them; and the want of that is enough to destroy that universal assent which must needs be the necessary concomitant of all innate truths: it seeming to me near a contradiction to say that there are truths imprinted on the soul which it perceives or understands not; imprinting, if it signify anything, being nothing else but the making certain truths to be perceived. For to imprint anything on the mind without the mind's perceiving it, seems to me hardly intelligible. If therefore children and idiots have souls, have minds, with those impressions upon them, they must unavoidably perceive them, and necessarily know and assent to these truths; which since they do not, it is evident that there are no such impressions. For if they are not notions naturally imprinted, how can they be innate, and if they are notions imprinted, how can they be unknown? To say a notion is imprinted on the mind, and yet at the same time to say that the mind is ignorant of it, and never yet took notice of it, is to make this impression nothing. No proposition can be said to be in the mind which it never yet knew, which it was never yet conscious of.

…Let us suppose the mind to be, as we say, a blank tablet (tabula rasa) of white paper, void of all characters, without any ideas; how comes it to be furnished? Whence comes it by that vast store, which the busy and boundless fancy of man has painted on it with almost endless variety? Whence has it all the materials of reason and knowledge? To this I answer in one word, from experience: in that all our knowledge is founded, and from that it ultimately derives itself.

6. FROM New Essays on Human Understanding[7] BY Gottfried Wilhelm von Leibniz

The question at issue is whether the soul in itself is entirely empty, like the tablet upon which nothing has yet been written (tabula rasa), as is the view of Aristotle and the author of the Essay (Locke), and whether all that is traced on it comes solely from the senses and from experience; or whether the soul contains originally the principles of various notions and doctrines which external objects merely awaken from time to time, as I believe, with Plato and even with the Schoolmen, and with all those who take in this sense the passage of St. Paul (Romans, 2:15) where he remarks that the law of God is written in the heart .... From this there arises another question, whether all truths depend on experience, that is to say, on induction and examples, or whether there are some that have some other basis. For if some events can be foreseen before any trial has been made of them, it is clear that we must here contribute something of our own. The senses, although necessary for all our actual knowledge, are not sufficient to give us the whole of it, since the senses never give anything except examples, that is to say, particular or individual truths. All examples which confirm a general truth, however numerous they may be, are not enough to establish the universal necessity of this same truth; for it does not follow that what has happened will happen again in the same way.

It would seem 'that necessary truths, such as are found in pure mathematics, and especially in arithmetic and in geometry, must have principles the proof of which does not depend on examples, nor, consequently, on the testimony of the senses, although without the senses it would never have occurred to us to think of them. This ought to be well recognised; Euclid has so well understood it that he often demonstrates by reason what is obvious enough through experience and by sensible images. Logic also, together with metaphysics and ethics, one of which forms natural theology and the other natural jurisprudence, are full of such truths; and consequently their proof can only come from internal principles, which are called innate. It is true that we must not imagine that these eternal laws of the reason can be read in the soul as in an open book, as the edict of the praetor can be read in his album without difficulty or research; but it is enough that they can be discovered in us by dint of attention, for which opportunities are given by the senses. The success of experiments serves also as confirmation of the reason, very much as proofs serve in arithmetic for better avoiding error of reckoning when the reasoning is long.

It seems that our able author claims that there is nothing potential in us and nothing even of which we are not at any time actually conscious; but he cannot mean this strictly, or his opinion would be too paradoxical; for acquired habits and the contents of our memory are not always consciously perceived and do not even always come to our aid at need, although we often easily bring them back to mind on some slight occasion which makes us remember them, just as we need only the beginning of a song to remember the song. Also he modifies his assertion in other places by saying that there is nothing in us of which we have not been at least formerly conscious. But besides the fact that no one can be sure by reason alone how far our past apperceptions, which we may have forgotten, may have gone, especially in view of the Platonic doctrine of reminiscence, which, mythical as it is, is not, in part at least, incompatible with bare reason; in addition to this, I say, why is it necessary that everything should be acquired by us through the perceptions of external things, and that nothing can be unearthed in ourselves? Is our soul, then, such a blank that, besides the images borrowed from without, it is nothing? . .. [T]here are a thousand indications that lead us to think that there are at every moment numberless perceptions in us, but without apperception and without reflections; that is to say, changes in the soul itself of which we are not conscious, because the impressions are either too slight and too numerous, or too even, so that they have nothing sufficient to distinguish them one from the other; but, joined to others, they do not fail to produce their effect and to make themselves felt at least confusedly in the mass.

FROM An Essay Concerning Human Understanding[8] BY John Locke

1. Concerning the simple ideas of Sensation, it is to be considered-that whatsoever is so constituted in nature as to be able, by affecting our senses, to cause any perception in the mind, doth thereby produce in the understanding a simple idea; which, whatever be the external cause of it, when it comes to be taken notice of by our discerning faculty, it is by the mind looked on and considered there to be a real positive idea in the understanding, as much as any other whatsoever; though, perhaps, the cause of it be but a privation of the subject.

2. Thus the ideas of heat and cold, light and darkness, white and black, motion and rest, are equally clear and positive ideas in the mind; though perhaps, some of the causes which produce them are barely privations, in those subjects from whence our senses derive those ideas. These the understanding, in its view of them, considers all as distinct positive ideas, without taking notice ofthe causes that produce them: which is an inquiry not belonging to the idea, as it is in the understanding, but to the nature of the things existing without us. These are two very different things, and carefully to be distinguished; it being one thing to perceive and know the idea of white or black, and quite another to examine what kind of particles they must be, and how ranged in the superficies, to make any object appear white or black.

7. To discover the nature of our ideas the better, and to discourse of them intelligibly, it will be convenient to distinguish them as they are ideas or perceptions in our minds; and as they are modifications oj matter in the bodies that cause such perceptions in us: that so we may not think (as perhaps usually is done) that they are exactly the images and resemblances of something inherent in the subject; most of those of sensation being in the mind no more the likeness of something existing without us, than the names that stand for them are the likeness of our ideas, which yet upon hearing they are apt to excite in us.

8. Whatsoever the mind perceives in itself, or is the immediate object of perception, thought, or understanding, that I call idea; and the power to produce any idea in our mind, I call quality of the subject wherein that power is. Thus a snowball having the power to produce in us the ideas of white, cold, and round-the power

[1] 2 Bertrand Russell. The Problems of Philosophy, Oxford: Oxford University Press. 1912.

[2] 4 Plato, from Theatetus, trans. F. M. Cornford, reprinted in The Collected Dialogues of Plato, ed. Edith Hamilton and Hunlington Cairnes, Bollingen Series, Princeton: Princeton University Press, 19BO, p. 61.

[3] 5 Rene Descartes, Meditations on First Philosophy. in The Philosophical Works of Descartes. trans. Elizabeth S. Haldane and G. R. T. Ros s, 1911. Reprinted with the permission of Cambridge University Press.

[4] 6 Rene Descartes, Meditations on First Philosophy, in The Philosophical Works of Descartes, trans. Elizabeth S. Haldane and G. R. T. Ross, 1911. Reprinted with the permission of Cambridge University Press.

[5] Rene Descartes. Meditations on First Philosophy. in The Philosophical Works of Descartes, trans. Elizabeth S. Haldane and G. R. T. Ross. 1911. Reprinted with the permission of Cambridge University Press.

[6] John Locke, An Essay Concerning Human Understanding. ed. A. C. Fraser, Oxford: Clarendon Press, 1894.

[7] "Leibniz's Rebuttal," from New Essays on Human Understanding. by Gottfried Wilhelm von Leibniz, trans. A. G. Langley. Reprinted by permission of Open Court Publishing Company, a division of Carus Publishing Company, Peru, IL. Copyright © 1949 by Open Court Publishing.

[8] John Locke, An Essay Concerning Human Understanding, ed. A. C. Fraser, Oxford: Clarendon Press, 1894.