Berkeley Epistemology


0.1  INTRODUCTION
Epistemology, being a branch of philosophy, is mostly interested in knowledge. All through the history of philosophy, philosophers have always pre-occupied themselves with questions mostly related to epistemology; how we come to know, the source of knowledge, the certainty of our knowledge claim, whether we can know for certain. All these are challenges that bother the epistemologists. Epistemology is therefore concerned with the scope, nature and extent of human knowledge. It also questions the certainty and reliability of human knowledge.
Epistemology has also been viewed as the study of justified beliefs, and it has truth as its major agenda. Berkeley was among the thoughtful figures that set out to search for a lasting panacea to this perennial problem which is as old as man. The crux of the matter is; do we really have knowledge at all? And if we do what is the source of our knowledge. The possibility of knowledge, search for knowledge transited from the Greek era of mythology to the critical and scientific investigation into the nature of the Cosmos. The hallmark of this intellectual dialectics is orchestrated in the views of Heraclitus on one hand, as the apostle of change and Parmenides on the other hand as the champion of unity, oneness, changelessness, and being.
Owing to doubt about the rationalist theory of knowledge, many philosophers have searched for a theory of knowledge consistent with ordinary human behaviour. Rather than rejecting the data we acquire through our senses in favour of some completely certain knowledge about a non-visible world, some philosophers have begun with our sense experience as the source and basis of what we know and have tried to construct an account of knowledge in terms of sense experience. This theory, which attempts to explain knowledge from the perspective of sensual experience, is called “empiricism”, and to this school, George Berkeley belongs.
Berkeley is very well known for his philosophical dictum esse est precipi (to be is to be perceived). Berkeley became the object of severe criticism and ridicule for with his dictum, he denies what seemed obvious to anyone. He had set out to deny the existence of matter. The potential consequence of his dictum is the annihilation of things when they are no longer perceived. Though Berkeley will respond that he does not take away existence by this theory but only declare the meaning of the world so far as he comprehends it.[1]
This paper shall not expound on the entirety of Berkeley’s philosophical ideas but our main focus, which shall also be the crux of this work is to probe into the very fabric of his epistemology. By this we refer to his ideas and understanding of perception as a source and justification of knowledge. We shall not claim to make an exhaustive research on this issue but our methodology shall be firstly to expose the different sources of knowledge as stipulated by philosophers of different epochs. After this we shall try to understand the idea of justification and the problem of perception with regard to our knowledge claim. This shall take us to expose briefly the Berkelian epistemology which shall naturally lead us to the crux of our work, that is, Berkeley’s theory of perception. At the end of this paper we shall make brief evaluation and then conclude.
1.0 Sources of Knowledge
There is no better and comprehensive way to explicate knowledge other than that which is related to its sources. This is in part why terms like perception, intuition and other generally recognized sources of knowledge have been treated extensively by epistemologists. These and other seeming sources of knowledge have also been recognized or accepted as all sources of justification. And they can serve as such even if justification is not entailed by knowledge.
If we look closely at the history of epistemology, we cannot but agree that the best candidates to bear the title “the classical basic sources of knowledge” to perception, memory, consciousness (or sometimes called introspection), reason (also called intuition).[2] However some writers have preferred to go the path of brevity and streamline them to just experience and reason or what we can also refer to as a priori and a posteriori respectively. Let us go further to consider these sources from what we can call basic sources and nonbasic sources of knowledge. We shall explore what it means for a source to be basic and some other conditions under which beliefs it yields constitutes knowledge. We can then consider what kind of source can be “nonbasic”. Paul Moser in his book The Oxford Handbook of Epistemology, gave some interesting analysis to this regard. 
Let us take it that a source of knowledge is something in the knower- let us say intuition and introspection- that yields beliefs concerning knowledge. Any source of knowledge which we shall accept as basic should be such that yields knowledge without any dependence on the operation of some other source of knowledge. Hence, might perceptually know that the clock says ten by virtue of seeing its face displaying that time; and might know by brief reflection that if two persons are first cousins, they share a pair of grandparents.  Paul suggests that it may seem that the perceptual knowledge of me knowing the time is dependent upon my memory, since my memory aids me in reading the time on the clock. Hence such perception cannot yield knowledge independent of memory, making it nonbasic.[3] It is true that perceptual knowledge of this kind depends on memory in a certain way. However let us consider this. Paul Moser contends that a being could acquire the concept needed for reading a clock at the very time of seeing one, and hence would not need to remember anything in other to form the belief that the clock says ten. Although perceptual knowledge depends in certain way on memory, neither the concept of perception nor perceptual knowledge is historical. Memory, according to him is historical, at least in the sense we are considering: it is impossible for one to remember something unless one has retained it in memory over some period of time.
Again, we can contend that perception is not a positively independent source of knowledge because it depends on consciousness. The idea is that, it seems (if not) impossible, that one should perceive without being conscious, hence perception cannot yield knowledge apart from the operation of another source of knowledge. Now, let us grant for the sake of argument that perception requires consciousness.  If this is so, then what we have is a kind of consciousness; a consciousness of external object. Paul Moser argues that we might simply grant that perception is perceptual consciousness and treat only of what he calls “internal consciousness” (consciousness of what is internal to the mind) as a source of knowledge distinct from perception. Internal consciousness if well understood, occurs when the objects is either internal to the mind just as images and thoughts are or abstract as in the case of numbers and concepts.
It may also seem plausible to threat consciousness as a kind of perception: external perception where the object is outside the mind and internal perception where it is inside.[4] But we can posit that abstract objects are not in the mind the way thought and sensation are. Since there is apparently a causal relation between the object of perception and the sensation constituting a perceptual response, it is not preferable to consider consciousness of this kind a kind of perception. Hence, we may take perception to be a partly causal notion. What we feel and experience sensually has a certain effect in us.
Memory can also be looked upon as a basic source of justification to our belief. We can surely be justified in believing something either on the basis of our remembering that it is, or on the basis of our having a clear memory belief that it is so, even if the belief is false. It is very necessary for us to acknowledge the status of memory as basic since it resides at the very core of our cognitive faculty. Whenever we remember something, we believe that it is so, because we have a certain belief and justification that our memory cannot mislead or present faulty fact to us except if we are deranged. But memory is not only a source of justification as to when I am able to represent in exact manner, a particular statement of a lecturer during examination. It is also a source of knowledge in so far as it is there to present a justification to our held beliefs. The sense of memory can present justification by virtue of the manner the proposition occurs to us. This normally occurs when we have forgotten our original, remembered justification such as when we witness a particular event that we have previously witnessed but had forgotten, we now have a memorial sense of it. [5]
Looking at memory as a capacity for preservation and not the creation of beliefs and knowledge, cannot be said to be a basic source of knowledge. But it is also very necessary in our knowledge claim because with it perceptual knowledge could not be amassed and used to help us build theories of human experience.
Now let us take a cursory view at reason in reference to a priori knowledge. Kant, in his Critique of Pure Reason, contends that “though our knowledge begins from experience, it does not follow that they all arise from experience.”[6] For, on the contrary, there is every possibility that our empirical knowledge is a compound of that which we receive through impressions, and that which the faculty of cognition supplies from itself.[7] Hence we are left to ask a question which does not require hasty but reflective answer; whether there exists a knowledge altogether independent of experience, and even of all sensuous impressions? Knowledge of this nature is called a priori, contrariwise to empirical knowledge which has its sources a posteriori, that is, through experience.
From Kant, we can infer that there is a difference between when something is said to begin from particular point and when something is said to have actually arisen from a source. In this light, we see the modern period in this struggle to inquire into the very foundation of knowledge- a basis on which all knowledge was built. And this inquiry is formally championed by Rene Descartes, who is fondly known for his Cogito ego Sum (I think therefore I am).[8]
However, one possible question we may likewise ask is: which is the true source of knowledge? How is it possible or rather how can we acquire knowledge that is independent of experience Since the rationalists claim that we can have certain knowledge that is independent of experience? This question is not that a simple one especially when we take a look at the changing nature of our world. This was what led Heraclitus to assert that everything is in a state of constant flux, hence nothing can be known; what exist is not Being but Becoming; change is the only reality. For it is not given to the mind of man to know what is at variance to itself. [9]
A changing being or a “becoming” cannot be said to be known either with the senses or with the mind construction since the change may somewhat be substantial hence changing the either being. But it is very obvious that Heraclitus was not talking about accidental change which will not affect the substratum or the substantial form of the being. Therefore, in the threshold to solving this problem, Descartes in his Meditations, embarked on his doubting enterprise. He doubted all possible knowledge and everything, demolishing all the pyramid of knowledge in order to arrive at a foundation upon which all other knowledge can be (or are) built.[10]     
2.0 The Problem of Perception
The problem of perception cannot adequately be comprehended if we fail to take a clear-cut historical excursion in the discourse of perception as a philosophical concept. Perception has undergone series of historical developments; and this development can be traced from the ancient period particularly Parmenides of Elea. Parmenides, with his distinction between ‘appearance and reality’ rejected the common sense notion of change. He avows that change is the confusion of appearance with reality, and hence change is simply an illusion.[11] He hinges his argument on perception, arguing and questioning our perceptual knowledge claims.
The sophists came into the scene and propounded that knowledge is relative. In this vain, knowledge for them is seen as perception. This is very well championed by Protagoras as we see him posits that knowledge is limited to our various perceptions and these perceptions will differ with each person.[12] He is very well known for his dictum, “man is the measure of all things.” Hence by positing this, it simply means that our knowledge is measured by what we perceive, and we all have our uniqueness in the manner we perceive. Therefore, since we cannot have sameness in perception, knowledge is simply relative.
Taking a brief glimpse at Theatetus, we see where Socrates convinces the young man Theatetus to discard the definition of knowledge as “nothing else but perception.”[13] For Theatetus opines that one who knows something is perceiving that which one knows, and so knowledge is nothing but perception.[14]
We can also see in the innatism of Plato, a blatant disregard of perception as knowledge. In fact he criticised the notion of perception as knowledge, though not discarding the possible acquaintance with material things which perception can furnish us with; for he argues that we perceive through our sense and not without them.[15] But Aristotle thinks it obnoxious that Plato should maintain a duplicate world. In fact he vehemently critiqued him on that. Forms, according to Aristotle is embedded in matter, hence there is not ‘matterless’ form and formless matter.[16] The state of knowledge cannot be innate in a determinate form, according to Aristotle, and also they cannot develop from other higher state of knowledge, but from sense perception.[17] A faithful disciple of Aristotle worth noting here, Thomas Aquinas, gave credence to sense perception as a source of knowledge. He opines that human mind originally has the potency to know and harbour knowledge but it has not innate ideas. Moreover, the knowledge the mind gains has its source in sense perception. [18]
Coming to the modern era, we see John Locke trying to resolve this problem of perception. Locke takes perception to be the first simple idea of reflection which comes about when the mind receives impression. Perception is, according to Locke, the first faculty of the mind which is also the simplest idea we have from reflection and it is called thinking in general.[19]
Having expounded and taken a short voyage through the philosophers of the ancient, medieval and modern epochs, to understand the problem of perception, it is therefore necessary and ad rem to have a brief insight into Berkeley’s epistemology. This is to aid comprehension and further the possibility of attaining the aim of this essay. 
3.0 Berkeley’s Theory of Perception
Berkeley’s epistemology is a very fascinating one especially when he starts to probe into the very idea of matter and sensible qualities. He posits that every sensible quality and things are ideas in the mind. And this sensible quality is either an idea imprinted on the sense or is perceived by attending to the passions and operations of the mind. Hence, every idea and thing, be it colour, love, book, car, tree, stone, figure and so on, exist in the mind.[20]
Looking at the beginning of Berkeley, one may confuse him for a rationalist. This may stem from his use of mind and elevation of the mind as the storehouse, font and harbour of sensible qualities and things. But this is not so. The manner Berkeley conceives of mind in his theory of knowledge is quite different form the way rationalists use it.  The rationalists associates the mind with the rational faculty that abstracts the object of reality. But this is not so in the case of Berkeley. Berkeley conceives of mind as a perceiving element that perceives reality. In his typical empiricism, he asserts
Besides all that endless variety of ideas or objects of knowledge, there is likewise something which knows or perceives them and exercises diverse operations, as willing, imagining and remembering about them. This perceiving active being is what I call mind, spirit, soul or myself.[21]

This led Berkeley to deny the existence of matter as a metaphysical substance, but he makes no mistake to deny the existence of physical objects like rivers and rock. He understands the existence of things based on the fact of their perceptibility, that is to say, a thing exist only when it is perceived either through a medium or in the mind. To substantiate this, a table upon which I write, I can say exist, that is because I see and feel it. And if I leave my study, I will say the table existed, meaning that if I were in my study I might perceive it or other spirit actually dos perceive it.[22] Berkeley contends that he does not argue against the existence of any one thing that we can comprehend, either by sense or reflection. According to him, the only thing whose existence is under the scrutiny of denial is what the philosophers call matter or corporal substance. This sort of denial does not do any damage to the rest of mankind who will never miss it.[23] This is what we now assume or rather ascribe as his basic thought on idealism and it is regarded sometimes and somewhat derisively called “immaterialism” or often times, subjective idealism.
In this statement above, we see an obvious vehement opposition and indignation Berkeley has against materialism. By implication, Berkeley denies the very possibility of inert, mindless, material substance. This description has some advantages over idealism in that it brings out Berkeley’s radical opposition to materialism; whereas the opposite of idealism is realism, and there are grounds for doubting whether Berkeley intended to deny the realist contention that in perception people become directly aware of objects that persist unchanged when they cease to be perceived. To state succinctly, Berkeley’s fundamental view was that for something to exist, it must either be perceived or it is the perceiving subject, that it, the active being doing the perceiving.
Joining in the thought voyage of Berkeley; we can interpret that for us to really know an object, we must first establish that such object really exist. In the third article of his Treatise, he wrote using a combination of Latin and English, esse is percipi, (to be is to be perceived).[24]  however, this proposition is most often slightly inaccurately attributed to Berkeley as the pure Latin phrase esse est percipi.[25] Nonetheless, the phrase is associated to him in authoritative philosophical sources.[26] In addition, for us to establish that an object of reality exist through perception of the mind, it is vital that the mind be conscious of the fact that, the object of intentionality which it is conscious of, exists. It is at this point that the mind can be fully convinced that it perceived the object. Contrariwise, Berkeley stated that, the moment the mind failed to be conscious of the object of reality of any idea, it is imperfectly unintelligible to believe such a thing exist.  
Berkeley does not consider abstraction as a philosophically non-issue, in fact he opines that abstraction is a concept people should not take seriously. He discards it by opining that if we say that objects of reality exist independently of the mind perceiving them, then it implies that what we have at the basis of the statement is the belief in doctrine of abstract ideas.[27] He rather sees abstraction as non-existing, for abstraction separate the object from its object. A plausible example of this is when we can perceive the smell of a soup but do not actually see the soup itself. Nonetheless, it seem impossible and difficult for one to adequately conceive an object without perceiving it.
However, in conception and imagination, one does not extend beyond the possibility of real existence or perception. Thus, just as it is impossible for one to feel anything without the actual sensation of that thing; so is it impossible to create a concept in my thought (or rather conceive in my thought) any sensible thing or object distinct from the sensation or perception of it. This takes us back to that ancient Aristotelian assertion, that nothing enters the intellect without first passing through the senses.[28]  There is a process of acquisition and ratiocination, which has to do with the senses giving raw data to the intellect to work with. In truth, the object and the sensation are the same and cannot therefore be abstracted from each other.[29] Therefore, it is difficult to demarcate, as an epistemic agent, the knowledge that is free from experience, from the knowledge acquired through experience. Even in the process of our speculation, there is (seemingly) an element of experience that filters into our experience.
One very interesting position of Berkeley, is the idea that nothing can exist without a mind perceiving it, or being the active mind perceiving. This led him to refute the Descartean Cogito Ego Sum. According to him, the claim cannot be arrived at through reason or abstraction alone, if such notion escapes the perception of an active mind. The problem with Descartes is that he separates mind perception and emphasises reason as a means to arriving at a reality. For the proposition “I think” is the proposition a solitary person attains by himself; a position from which it is impossible to regain solidarity with other humans who exist outside of the self, because the ego cannot reach them through the cogito.[30] This eventually lands to the problem of other minds.
4.0   Critique of Berkeley’s theory of Perception
Berkeley spent most of his life working and trying to prove that matter in the sense in which the word is generally used, does not exist and also that abstraction, as rationalism emphasises, does not exist. However plausible these may seem to his followers, a lot of philosophers have found his position very enormous, faulty and filled with incoherent logical anomalies. A crystal-clear example of this is his vehement denial of the most obvious phenomena to the senses; Matter. Some philosophers of his time did not find his position appealing in any ramification. They posited that matter is all that can be said to have any real existence in the entire universe. In their judgement, whatever happens in our world can be traced and adjudged based on its connectivity to matter.[31] However the problems many philosophers have with him seem to have their hub on this famous claim of his that “whatever is to be known and exist must be perceived by the mind.”[32]
G. E. Moore finds fault in Berkeley’s position concerning perception. According to him, Berkeley makes perception the criterion for existence which he considers a misplacement of value, and a category mistake. Berkeley argues that, instead of the esse of the perceived datum to consider in its percipi; the datum of which I am aware necessarily has a status not reducible to my awareness of it.[33] This assertion is rejected by Moore who posits that, whether there is a mind perceiving things or not, they exist always, as against Berkeley who says that things do not exist without the mind that perceives them.
G. E. Moore claimed that idealists including Berkeley fail to distinguish between the act and object of sensation.[34] Moving further, Moore posits that both a sensation of idea and its object exist. What this means is that to say that one exists, is to say that the other exists too. Hence, Moore asserts that
…when therefore, Berkeley, supposed that the only thing of which I am directly aware is my own sensation and ideas, he supposed what was false… I am as directly aware of the existence of material things in space as of my own sensations; and what I am aware of with regard to each is exactly the same- namely that in one case the material thing, and in the other case my sensation does really exist.[35]

Although Moore hints that some things like toothache cannot exist except when it is felt (perceived), he however notes that the moon certainly can exist without being perceived.[36] In other words, Moore disagrees with Berkeley that perception is the mark of existence.
Another problem we can see in based on his doctrine of his immaterialism, which is a necessary consequence of his notion on perception. His doctrine of immaterialism leads to solipsism, that is, the view that the universe is nothing but minds. Solipsism denies the existence of other minds or those things that are not perceived by it. In other words, as far as one can tell, is nothing but myself, my mind and its ideas.[37] If we now accept Berkeley position, which is tantamount to accepting solipsism, it would be difficult to communicate our ideas to one another.[38] This problem did not escape Berkeley when he was making his reflection. He saw the difficulty and so to escape it, he postulated a multiplicity of spiritual substances and also the existence of infinite mind, God.[39] Berkeley’s recognition and appeal to God does not bring him any closer towards solving this problem because, this solution cannot hold when it is presented to the atheists and those who do not attribute omnipresence to God. The underlying flaw we see, therefore, in his doctrine of immaterialism is the inability to properly show either the existence of God, or the non-existence of matter without requiring God’s existence.
According to David Hume, Berkeley’s acceptance of spiritual substance and denial of material substance is contradictory. Since Hume is a thorough going empiricist, he holds that Berkeley’s spiritual substance cannot be empirically verified, hence, it does not exist. For David Hume;
If it is possible to have a notion of the soul of God, why is it absurd to have another notion of matter? If the words, material substance represents a meaningless expression, and unintelligible abstraction, surely, the same applies to infinite or spirit.[40]

For Immanuel Kant, Berkeley expounded what he calls mystical idealism, which makes the whole sensible world into a mere illusion.[41]In other words, Berkeley’s theory definitely reduces material things to subjective states and eventually sheer illusions.
It is also very interesting to bring the rebuttal of Russell here, who in his critique of Berkeley asserts;
It is impossible for a nephew to exist without an uncle; now Mr A is a nephew; therefore it is logically necessary for Mr A to have an uncle. It is of course necessarily necessary given that Mr A is a nephew, but not from anything discovered by analysis of Mr A. So if something is an object of the senses, some mind is concerned with it; but it does not follow that the same thing could not have existed without being an object of the senses.[42]
Ludwig Wittgenstein argues that since perception from Berkeley’s point of view is subjective, we would have only private experience, and not only would they be incommunicable, we could not describe or speak about them even ourselves, for the use of language involves rules which are common and have to be established and checked with respect to public objects. Berkeley’s argument denies the ontological existence of physical realities. Hence, his contention that without sense, the world cannot exist is repugnantly absurd.[43]
From the foregoing, we can say that Berkeley admitted the existence of matter in his attempt to deny it. For he admits its existence when he agrees that what we perceive exist. His problem, however, is that instead of discussing the existence of what he perceives, he only focused on discussing the nature of this existence.
5.0 Evaluation and Conclusion
We have attempted in the course of this essay to expose the controversial theory of Berkeley concerning perception. As a recap and thus evaluation, Berkeley denies the existence of material substances, all that is, is mind. For him, matter is a meaningless term. Hence, all we call body or in his word, matter, are just ideas in the mind, which ceases to exist when the mind stop perceiving them. This he summarises in his famous dictum esse est percipi “to be is to be perceived”. if we are to take Berkeley’s position serious, we shall assume this paper and other things we know as material realities as illusions. In other words, we see ideas, we walk on ideas, we feed on ideas, and lots of other instances. By saying that the esse of things consists in their percipi, Berkeley on one hand, postulated an important theme in philosophy, that is, he gave primacy to the mind as a seat of cognition. He however went to the extreme when he denied the existence of matter. His view also known as subjective idealism, alienates the world as we know it.
Berkeley may have postulated this theory to refute atheists and those who worship objects, but the way he approached it left the world empty, hence making his theory faulty. As a result of the inconsistencies of his theory, many criticisms were uttered (as we have presented in the last part of this essay), against his theory. And through this we can see that perception, although a veritable source of knowledge, does not exhaust and is not an independent basic source of knowledge. All this essay has made us to see is the incoherencies in assuming a contrary position. Hence, knowledge is broad and its sources should neither be streamlined and limited to one nor be exaggerated to lots of incomprehensible sources.





[1] Cf. Samuel Enoch Stumpf, Philosophy: History and Problems (New York: McGraw-Hill Publishing Company, 1998), p. 274.

[2] Cf. Paul K. Moser, The Oxford Handbook of Epistemology (London: Oxford University Press, 2002), p. 72

[3] Cf. Paul K. Moser, The Oxford Handbook of Epistemology (London: Oxford University Press, 2002), p. 72
[4] Cf. Paul K. Moser, The Oxford Handbook of Epistemology (London: Oxford University Press, 2002), p. 73

[5] Cf. Robert Audi, Epistemology: A Contemporary Introduction to the Theory of Knowledge (UK: Routledge, 2003), p. 1

[6] Cf. Immanuel Kant, Critique of Pure Reason, Transl. F. Max Muller (New York: Doubleday & Company, Inc. 1966), p. 41.
[7] Kant believes that the faculty of cognition has the capacity to supply impressions or “knowledge” to impressions which we receive from the outside space, this is very well explain in his discourse concerning Space and Time.
[8] Cf. Descartes, “Rules for the Direction of the Mind”, 425; in John Cottingham et al, The Philosophical Writing of Descartes Vol. 1, (new York: Cambridge University Press, 1985), p. 48.
[9] Cf. Frederick Copleston, A History of Philosophy, Vol. 1 (New York: Newman Press, 1946), p. 59.
[10] Cf. Rene Descartes, Meditations on First Philosophy. Transl. by Elizabeth S. Haldane (new York: Cambridge University Press, 1996), Meditation 1, Article 1
[11] Cf. Samuel E. Stumpf, Philosophy: History and Problems 4th ed. (New York: McGraw-Hill 1994), p. 17.
[12] Cf. Samuel E. Stumpf, Philosophy: History and Problems, p. 32.
[13] Plato, Theatetus, H. N Fowler (Transl.), 151E
[14] Cf. Bertrand Russell, History of Western Philosophy (London: George Allen and Unwin Ltd, 1961), p. 163.
[15] Cf. Bertrand Russell, History of Western Philosophy, p. 163.
[16] Cf. Bertrand Russell, History of Western Philosophy, p. 91.
[17] Aristotle, Posterior Analytics, Book 2, Ck 19, para. 100a
[18] Cf. Frederick Copleston, A History of Medieval Philosophy, Vol. 2 part 1, (New York: Image Books, 1962), p. 112.
[19] Cf. John Locke, An Essay Concerning Human Understanding (London: Oxford University Press, 1975), p. 525.
[20] Cf. George Berkeley, A Treatise Concerning the Principles of Human Knowledge (Illinois: The Open Court Publishing Company, 1963), part 1, article 1
[21] George Berkeley, A Treatise Concerning the Principles of Human Knowledge, part 1, article 1
[22] Cf. George Berkeley, A Treatise Concerning the Principles of Human Knowledge, part 1, article 3.
[23] George Berkeley, A Treatise Concerning the Principles of Human Knowledge, part 1, article 35.
[24] George Berkeley, A Treatise Concerning the Principles of Human Knowledge, part 1, article 3.
[25] Cf. Robert Forgelin, Berkeley and the Principle of Human Knowledge (London: Routledge Press, 2001), p. 27.
[26] Cf. Lisa Downing, “George Berkeley,” in The Stanford Encyclopedia of Philosophy edited by Edward N. Zalta, http://plato.stanford.edu/archives/spr2013/entries/berkeley/ Retrieved 29, April 2016. 
[27] Cf. George Berkeley, A Treatise Concerning the Principles of Human Knowledge, part 1, article 4-5.
[28] Cf. Aristotle, “De Anima”, Bk. 1, 402b, 20-403a, in The Basic Works edited by Richard Mckeon (New York: The Humanities Press, 1961).
[29] Cf. George Berkeley, A Treatise Concerning the Principles of Human Knowledge, part 1, article 5.
[30] Cf. Jean-Paul Sartre, Existentialism from Dostoyevsky to Sartre. Transl. by Walter Kaufman (New York: Cambridge University, 1946), p. 1a
[31] Cf. Charles H. Patterson, Berkeley’s Major Philosophical works, (Nebraska: Cliffs Notes, Inc., 1972), p. 9
[32] George Berkeley, A Treatise Concerning the Principles of Human Knowledge, part 1, article 2.
[33] Cf. Kenneth T. Gallagher, The Philosophy of Knowledge New York: Sheed and Ward, 1996), p. p. 29.
[34] Cf. E. D. Klemke, The Epistemology of G. E. Moore (Evaston: North Western University Press, 1967), p. 112.
[35] G. E. Moore, Philosophical Studies (London: Routledge & Kegan Paul Ltd, 1922), p. 30.
[36] Cf. G. E Moore, “A Reply to My Critics” in The Philosphy of G. E. Moore ed. by Paul A. Schilpp, 2nd ed. (New York: Tudor, 1952), p. 653.
[37] Richard Popkin & Avrun Stroll, Philosophy Made Simple (USA: Doubleday Dell Publishing Group Inc., 1969), p. 294.
[38] Cf. O. A. Oyeshile, “George Berkeley’s Idealist Critique of Materialism: The Existential Consequences” in West African Journal of Philosophical Studies Vol. 5, (December 2002), p.90.
[39] Cf. George Berkeley, “principles of the Human Knowledge” Reproduced by R. J Hirst (ed.) in Perception and the External World (New York: Macmillan Company, 1965), p. 255.
[40] Cf. Maurice Conforth, Science and Idealism (New York: International Publishers, 1947), p. 51.
[41] Cf. Immanuel Kant, Prolegomena to Any Further Metaphysics (Cambridge: Cambridge University Press, 1997), 4:292.
[42] Bertrand Russell, History of Western Philosophy (London: George Allen and Unwin Ltd., 1965), p. 593.
[43] Cf. Celestine Bittle, Epistemology: Mind and Reality (New York: Bruce Publication Company, 1936), p. 153.

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