Wednesday, May 25, 2016

Hobbes and Hume on the Senses: a Response

This essay is a follow-up to “The Perceptual Level as Given.” It will discuss a philosophical school that tried to answer the question of what the mind starts with: the sensualists/empiricists. The bulk of this essay will be an extended presentation of the sensualist approach of consciousness and knowledge as expounded by key sensualists like Hobbes and Hume. That section will be followed by a couple of my own problems with sensualism as they relate to the perceptual level of consciousness. (My issues with the sensualist view of the conceptual level will have to wait until I work through the inductions of concept-formation. I’ve also modernized the words in Hobbes’ and Hume’s quoted statements.)

Hobbes and the “Great Deception of Sense”

Thomas Hobbes (1588-1679) discusses his view of perception in his magnum opus Leviathan, which contains the essentials of his whole philosophy. (It’s also discussed in his Elements of Philosophy Concerning Body.)

Hobbes’ Materialism and Empiricism

On the theory of metaphysics, Hobbes is a complete materialist. All that exists is material bodies in 3 dimensions, with magnitudes of length, breadth, and depth in motion, interacting with other bodies.
The world, (I mean not the Earth only, that denominates the lovers of it worldly men, but the universe, that is, the whole mass of all things that are) is corporeal, that is to say, body ; and has the dimensions of magnitude, namely, length, breadth, and depth: also every part of body, is likewise body, and has the like dimensions; and consequently every part of the universe, is body ; and that which is not body, is no part of the universe: And because the universe is all, that which is no part of it, is nothing[…] (Leviathan, “Of the Kingdom of Darkness,” Ch. 46)
In the Hobbesian epistemology, all knowledge is based on the senses; there are no innate ideas. “(For there is no conception in a man’s mind, which has not at first, totally, or by parts, been begotten upon the organs of sense.) The rest are derived from that original.” (ibid., “Of Man,” Ch. 1) By “that original,” he means sensations, and by “the rest,” he means imaginations, memories, emotions, and even our ideas. Thus Hobbes is also a thorough empiricist. Empiricism is basically the theory that sense-experience is the source of all knowledge, including all ideas.

Here is how he combines his materialism with his empiricism. Sensations are motions in the brain (and heart). Sensations also give us awareness of images which suggest an external world. This can occur because of the interaction of the external objects with our sense organs, which causes us to directly experience the inner content of our brains: the various sensations. (ibid., “Of Man,” Ch. 1, pp. 11-12) “The cause of sense is the external body or object, which presses on the organ proper to each sense, either immediately, as in the taste and touch; or mediately, as in seeing, hearing, and smelling[…]” (ibid., “Of Man,” Ch. 1, p. 11) This is known as the “causal theory of perception”: reality is merely the cause, not the object, of our perceptions, and the object of perception is taken to be some inner, mental experience or sense.

He also held that there are two sorts of qualities that our senses detect. The first sort are the properties that actually belong to the bodies themselves, which philosopher John Locke would later call the “primary qualities.” “The definition, therefore, of body may be this, a body is that, which having no dependence upon our thought, is coincident or coextended with some part of space.” (Elements of Philosophy Concerning Body, “Of Body and Accident,” Ch. 8, section 1) So our senses allow us to perceive figure/shape, size, number (of things in a group), and motion, which are considered to be the properties of bodies.

The “secondary qualities” that Hobbes accepts (and that Locke also named as a group) are the familiar sensations that we associate with the five senses. “For light and colour, and heat and sound, and other qualities which are commonly called sensible, are not objects, but phantasms [images or ‘fancies’] in the sentients.” (ibid., “Of Sense and Animal Motion,” Ch. 25, section 3) The sensory qualities are “in us,” they are images in our minds, not in the objects which cause them. On Hobbes’ view, the real world is invisible, odorless, temperature-less, texture-less (neither “smooth” nor “rough”), tasteless, inaudible, etc.

Hobbes believes that some of our experiences are similar to or match reality, namely the experiences of the primary qualities. The secondary qualities do not.
[…][W]hatsoever accidents or qualities our senses make us think there be in the world, they be not there, but are seemings and apparitions only; the things that are really in the world without us, are those motions by which these seemings are caused. And this is the great deception of sense[…]” (The Elements of Law Natural and Politic, Ch. 2, Sect. 10)
This view that (some) sensations can be similar to what’s really out there is the “representative theory of perception.” In this theory, perception is just another kind of looking inward into one’s own consciousness, just another process of introspection. Reality is known by inference from our introspections/inner awareness of qualities.

Sensualism

Eventually new stimuli striking our sense organs cause prior sensations to decay, and these become “images,” “memories,” “concepts.” “This decaying sense, when we would express the thing itself, (I mean fancy itself,) we call imagination.” (Leviathan, “Of Man,” Ch. 2, p. 14) Memory is imagination in the context or perspective of it being a sense that is fading or being obscured over time. (ibid.) Most crucially, a thought is an imagination that arises in us by means of words or other “voluntary signs.” (Ibid. p. 18) Thus even ideas are really just perceptions or images of senses. Such a view is the conclusion of the position known as “sensualism.”

Sensualism is the view that all mental phenomena, such as memories and imaginations, are really sense perceptions. Man has only one faculty of awareness, sense-perception, and all other “faculties” are merely forms of it. Therefore on this theory the process of gaining knowledge by reason is essentially the same as gaining it by sense-perception.

Hume and the “Cement of the Universe”

David Hume (1711-1776) explains his philosophical views in principally three books. The first was A Treatise of Huma Nature (Treatise), the second was An Enquiry Concerning Human Understanding (EHU), and the third was An Enquiry Concerning the Principles of Morals. (Treatise will be cited as book, part, section, and paragraph, and EHU will be cited as section, part and paragraph.)

Rather than having a fleshed-out, coherent philosophical system, Hume is known as the arch-skepticist in philosophy. His system begins with an unmitigated empiricism, but the principles that Hume adopts slowly demolish the most perennial positions held in philosophy.

Humean Empiricism

He begins along the lines of John Locke, whose views are assessed and attacked relentlessly in Hume’s works. Our perceptions come in two forms: impressions and ideas. Impressions are our sense experiences and emotions, whereas ideas are copies of impressions which are conveyed by our thoughts, a thesis known as the “copy principle.” The difference between ideas and impressions is merely their force or liveliness or vivacity: take an impression/sensation of a “chair,” make the impression dull and faded and you now have an idea of “chair.” (Although technically a “chair” would be a complex impression for Hume, consisting of the qualities of the chair like “size” and “shape” bundled together.) Our ideas can never become as vivid as their impressions, so impressions are always the original perceptions and ideas simply copy or represent/resemble them.

Ideas in Hume’s view come in two forms: ideas of memory and of imagination. Memories are somewhat lively ideas and are mainly preserved in the order that the original impressions reached a person’s “mind.” (The reason why I put “mind” in quotes should be become clearer at the end of this discussion of Hume.) Imaginations are quite different: they have no vivacity and thus are “perfect idea[s].” (Treatise 1.1.3.1) Imagination can combine simple ideas to make complex ones, and separate them back into simple ideas and generally rearrange them as it pleases. By nature’s grace, three ways of associating ideas generally operate on our imaginations: resemblance, contiguity (in time or place), and cause and effect. (Otherwise the imagination would run wild joining and separating ideas with no rules to restrain it.) These associating principles are the key to understanding how we come to have beliefs and organize our knowledge; their importance is symbolized by Hume’s claim in the book’s Abstract that these principles are “to us[…]the cement of the universe.” [Italics mine.]

Just from this much, we can see some similarities and differences with Hobbes. They both reject the notion of “innate ideas” and they both agree with empiricism that knowledge begins with sensory perception. And they’re both sensualists: ideas are just faded, decaying, less lively, less forceful sensory or emotional impressions. Both agree that imagination and the capacity to form images is crucial to human nature, affecting our sensations, memories, and thoughts. Hobbes believes that memories and thoughts are imaginations, and that the secondary, sensible qualities (color, sound, etc.) are phantasms, images. And Hume states: “The memory, senses, and understanding [i.e. thoughts] are, therefore, all of them founded on the imagination, or the vivacity of our ideas.” (Treatise 1.4.7.3, word in the bracket is mine.) However, Hume is more explicit and thorough in his sensualism than Hobbes, and is thus able to use this theory to undercut quite a few of Hobbes’ views. (Not that Hume directly challenges Hobbes, but that Hobbes along with Locke are part of the “modern philosophy” that Hume generally criticizes in the Treatise.)

Hume Against the “Modern Philosophy”

On Hume’s logic, the causal theory of perception is thrown out. The causal theory presumes that unperceived objects cause the perceptions that we experience. But causal reasoning for Hume is based on experiences of the constant conjunction of ideas, and there’s never a perception of an unobserved object, which means there can be no such idea either (the copy principle). Due to this, it is “impossible, therefore, that from the existence or any of the qualities of the former [perceptions], we can ever form any conclusion concerning the existence of the latter [objects], or ever satisfy our reason in this particular.” (Treatise 1.4.2.47, words in brackets mine.)

The same goes for the representationalist theory of perception. There is no basis to infer the existence of unperceived objects, let alone their resemblance to our perceptions. The reason why philosophers developed the theory is because of our overactive imaginations; we merely imagined that these things we call “objects” cause the perceptions we have, and so our imaginations completed the union of our ideas by making us think that the objects must also resemble the perceptions. (Treatise 1.4.2.54-55)

Another view that Hume attacks from the modern philosophy is the primary vs. secondary quality distinction. Crediting George Berkeley’s argument against the distinction, Hume demonstrates that without the ideas of the secondary qualities, there is no way to conceive of any of the purported primary qualities. He remarks that “after the exclusion of colours, sounds, heat and cold from the ranks of external existences, there remains nothing, which can afford us a just and consistent idea of body.” (ibid., 1.4.4.10 and EHU Part I, Section 12) If the secondary qualities are merely perceptions/images of the mind, and it turns out that the primary qualities can’t exist or be understood without the secondary ones, then Hume declares “nothing we can conceive is possessed of a real, continued, and independent existence; not even motion, extension and solidity, which are the primary qualities chiefly insisted on.” (Treatise 1.4.4.6)

The “Malady” of Skepticism

Despite these differences with the modern philosophy, Hume takes his version of sensualism to unprecedented depths of skepticism.

He theorizes that causation or necessary connection is really just the mind’s custom of uniting impressions or ideas of objects that appear to follow one another. The only relevant impression is not one of necessity, but “that propensity, which custom produces, to pass from an object to the idea of its usual attendant. This therefore is the essence of necessity.” Necessity is invented by our imaginations in that our minds connect impressions and ideas so that we believe that one must result from the other, and we mistakenly ascribe causal relations to them. (ibid., 1.3.14.21-22)

The external world is also considered to be fiction dreamt up by the imagination in Hume’s view. The mind’s experiences are perceptions, and all ideas are derived from these perceptions that were previously experienced. Logically then there is nothing that can be conceived that is “specifically different” from a perception, such as external objects or a world. We try to conceive of objects “out of ourselves as much as possible,” but “we never really advance a step beyond ourselves.” We end up thinking in certain ways about only our own perceptions, which are restricted to our imaginations. (ibid., 1.2.6.8)

Moreover, the senses never give us an idea of unperceived, distinct existing objects. (ibid., 1.4.2.11) Neither does reason. (ibid., 1.4.2.14) Rather, we observe objects in a temporary, discontinuous manner, but our minds are put into a disposition similar to when we look at an object without interruption. Since the separate, interrupted perceptions of an object resemble each other, the mind confuses the succession of perceptions with strict identity (i.e., an uninterrupted perception of an object). We customarily treat interrupted perceptions as identical, and this is due to our memories connecting past impressions with present ones, causing us to feign a continued existence of objects. And since the memories are lively, it causes us to believe this fiction of bodies that continue to exist even when unperceived. So the notions of “mind-independent” objects and of “identity” are also simply figments of our imaginations. (ibid., 1.4.2.32-43)

His analysis of the external world and of mind-independent objects leads to the view that every object is just a bundle of qualities. Hume for instance brings up the “colour, taste, figure, solidity, and other qualities combined in a peach or melon.” His view is that objects like these are not simple, but rather a composition of the aforementioned qualities and others like them. We commonly think of objects as simple, but our reasoned view is that they are made of distinguishable qualities; to reconcile these views our imaginations create the idea of “substance,” an “unknown something” (as Locke called it) which becomes a principle of union/cohesion for the qualities, such that we think that an object is simply one thing, in spite of its diverse qualities and composition. (ibid., 1.4.3.5) But remember that there’s no “identity” or “causation,” so it just so happens that objects are currently bundled the way that they are; since we can use the imagination to distinguish them as distinct qualities, they can exist apart from other qualities and even from a “substance” or entity. (ibid., 1.4.3.7)

Lastly, even the notion of the mind/soul or one’s self cannot withstand Hume’s inquisition. Hume’s philosophical view is that there is no “self” because ideas are copies of impressions, and there simply are no impressions of the self. The self is supposed to be an invariable amalgam of perceptions, but no impression is constant and invariable, and so there never could be an invariable self-impression, and thus “there is no such idea.” He attempts to search for this “self” but as a sensualist, all he finds are particular perceptions, “of heat or cold, light or shade, love or hatred, pain or pleasure.” He can never observe just himself alone; every experience has some kind of perception as its content. Rather, every self is just a “bundle or collection of different perceptions, which succeed each other with an inconceivable rapidity, and are in a perpetual flux and movement.” He likens the mind to “a kind of theatre, where several perceptions successively make their appearance; pass, re-pass, glide away, and mingle in an infinite variety of postures and situations.” (ibid., 1.4.6.2-4)

Further, the self is thought by Hume’s opponents to possess a “perfect identity and simplicity,” but he dismantles both of these assumptions. Our memory produces resembling perceptions, but it can’t fill in all of our perceptions’ gaps. However, our memory discovers the self incidentally by supplying the resembling perceptions, causing connections between our “minds” and our perceptions, such that we suppose that our self or personal identity continued to exist even in times when we cannot remember. (ibid., 1.4.6.20) The imagination’s relations of resemblance (in the form of memory) and causation cause us to create a fiction that withstands the changes to one’s successive perceptions and disposition. The fiction of an identical self masks the mental changes with “the notion of the soul, the self, and substance.” (ibid., 1.4.6.6, 19-20) As for its simplicity, he wrecks that idea in a similar fashion to the idea of unperceived, distinct objects: we confuse simple objects with the closely-related qualities of complex objects. We become confused by our various perceptions and tendencies when compared to our belief that something subsists through the changes in perception, and thus we create a fictitious principle of union of the “self” or the “mind” to overcome the contradiction. (ibid., 1.4.6.22) In short, the mind or self is neither “simple” nor “identical,” despite our imagination’s propensity to make us think that that’s the case. (ibid., 1.4.6.4)

So after such a sensualist deconstruction, what’s left to constitute his philosophy? Virtually nothing, and the resulting skepticism is for him “a malady, which can never be radically cured, but must return upon every moment, however we may chase it away, and sometimes seem entirely free from it.” He feels that the only way of counteracting this sense of hopelessness is “carelessness and inattention.” (ibid., 1.4.2.57) He has these beliefs and ideas without any means of determining their truth or falsehood. Hume seeks to dispense with all beliefs due to human reason’s flaws, as he feels that a given belief’s truth is just as probable as any other. He bemoans: “I am confounded with all these questions, and begin to fancy myself in the most deplorable condition imaginable, environed with the deepest darkness, and utterly deprived of the use of every member and faculty.” To escape from this philosophical nightmare, he doesn’t create some new way of thinking, but merely returns to the common, lazy thinking of common people with their imagined assumptions of the external world, the self, identity, causation, and all of the other ideas he explained away or demolished earlier. (ibid., 1.4.7.8)

He summarizes his dismal view of reality in an alarming observation:
All events seem entirely loose and separate. One event follows another; but we never can observe any tie between them. They seem conjoined, but never connected. And as we can have no idea of any thing which never appeared to our outward sense or inward sentiment, the necessary conclusion seems to be that we have no idea of connection or power at all, and that these words are absolutely, without any meaning, when employed either in philosophical reasonings or common life. (EHU, Section 7, part II, paragraph 1)
A Response to Sensualism

It is still premature to fully address all of my issues with sensualism, and more fundamentally the entire conflict between the empiricists and the rationalists. I’ll now focus on two essential points in the sensualist school: the importance they place on the faculty of imagination for its role in generating knowledge, and the sensualist assumption that perception is the result of a combination of sensations and/or sense qualities.

Imagining Things Differently

An interesting problem in the sensualist tradition is a failure to fully differentiate between our mental faculties. Hobbes thought that imaginations were just decayed sense experiences, and that memories and ideas are only forms of the imagination. Hume believed that imagination was the base of all faculties, even reason and the senses. I have major problems with this sensualist view of imagination, particularly the “resemblances” Hobbes and Hume saw between the other faculties and the imagination which causes them to attribute any type of identity amongst them.

Imagination is the ability to arrange and rearrange stored information, experiences, ideas, and generally any mental content through one’s will. It can also operate subconsciously, such as a person seeing a nearby bear, and subsequently feeling fear after imagining being mauled by it. Most striking is its ability to give us dreams and nightmares when we are asleep. It is not restrained by rational considerations or even realistic notions of causality, and so an innumerable number of events can be imagined that clash with reality, like imagining that a camp fire would not melt a scoop of ice cream very close to it.

Perception is a causal interaction between the perceiver and the perceived, in which the perceptual systems must relay the information that they receive in the forms available to them, following inexorable physical and psychological laws. We have a degree of control over when and how we use our perceptual systems, such as when we close our eyes to cease our sight, or plug our nose to stop our sense of smell. However, the nature of the perceptions and the means of processing for those systems are outside of our volition. Perception cannot ignore reality or mischaracterize it, as imagination is easily capable of. Perception cannot represent or misrepresent reality, but imagination can.

Perceptions and sensations are not images in the mind, but rather a mind’s awareness of something external interacting with its sense organs. Imaginations are entirely different phenomena than so-called “decayed sense perceptions.” “Decaying” perceptions would be more like after-images and noise-induced ringing ears, but even these aren’t imaginations. Rather, those phenomena are effects of over-exposure to certain stimuli. Imaginations do not require a stimulus from the environment; a person with sufficient information stored in their subconscious can imagine virtually whatever they want with absolutely no stimulus from the environment. Perceptions always require a stimulus because they are the results of a continuous process of interactions between a being’s perceptual systems and the environment.

More fundamentally, perception and imagination are dissimilar because imagination is not a cognitive faculty. Cognition according to the online Oxford dictionary is, “the mental action or process of acquiring knowledge and understanding through thought, experience, and the senses.” Cognitive actions are performed by animals and us, and they serve to give us information about reality. (Binswanger, How We Know, p. 57) Cognition takes the forms of sensations, perceptions, conceptions, thoughts, logical arguments, and even memories insofar as one is identifying something in one’s recollections (instead of recalling something just because one is capable of doing it). These acts of awareness are determining to some extent the identity of something, from the simplest sensation of a flatworm to the most complicated theorizing of a brilliant scientist. (ibid.)

The faculty of imagination can sort stored events and information in ways that are like and unlike how the world actually is, as well as how one originally experienced the events or gained the information. A person could imagine a horserace ending differently than it did in reality, or even fantasize about a world filled with lush and humid jungles in contrast to our world with its plethora of varied environments. But none of these imaginative activities are cognitive because images are not directed towards anything in reality; images have no independent connections to objects in the world. Imaginations are experiences that animals and humans produce, but they have nothing to do with gaining information about reality. Similarly, emotions are reactions to some content or information (perceived or imagined), but like imaginations they are merely experiences, not cognitive actions. (ibid.)

Considering all of this, I would say that the sensualists vastly overstate the capabilities of the imagination.

The Problematic “Atomic” View of Perception

Another theme of the sensualists is that perceptions are not a simple form of awareness, but are necessarily complex. Hobbes (and Locke) thought that we perceive objects which were combinations of the primary and secondary qualities, although the secondary qualities were presumed to be deceptions by Hobbes. Hume considered the “colour, taste, figure, solidity, and other qualities combined in a peach or melon.” Perceived objects are bundles of sensations of qualities for Hume, and we merely imagine that there are coherent, unified objects which are simple.

On the contrary, as philosopher Harry Binswanger eloquently puts it, “[p]erception is a unitary phenomenon; it does not have sensations or anything else as components.” (ibid., p. 64) Neither the brain nor the understanding/intellect constructs perceptions out of elemental “sensations.” Sensations and perceptions have physical causes, and these physical systems can be analyzed into parts, but this cannot be done for the sensations and perceptions themselves. States of consciousness have no parts as such; experiences are not broken up in reality. We experience our perceptions all together without breaks, despite the experience having distinguishable sense modalities (e.g., taste, touch, sight). Even from the start, the perceptions are unitary. One day a toddler (for instance) may realize that he possesses several sense modes such as sight and hearing, but he does not start with clearly discriminated modalities that he must put together into a combined experience. The experience starts as and remains “a single whole.” (ibid., p. 65)

As Binswanger wisely reminds us, material objects have parts that compose them: a chair is a combination of atoms, and conversely, atoms arranged in a certain way simply constitute a chair. Any physical object is a certain arrangement of atoms. “But consciousness is not matter.” (ibid., p. 64, italics mine) There are no “atoms” of consciousness, no components of awareness as such, which means that it’s an error to presume that sensations are “atoms” which compose perceptual awareness. There simply are no awareness or consciousness “atoms.”

Contra Hume, the perception of a melon is not a combination of our sensations of its figure, solidity, taste, texture, etc. Perception is of the melon against the backdrop of the table it’s resting on and the room spread out around the melon, table and the observer. There are no such sensations or atoms of awareness. We can analyze a perceptual experience and abstract out aspects or dimensions of the experience, like a sound’s pitch or the roughness of a fabric. But the mistake that Hume and others make is that they regard these aspects as individual “sensations,” when they should be more properly regarded as “sensory qualities” (ibid., p. 65). The sensory qualities are what we abstract out of our perceptual experiences, but the whole perceptual awareness of entities is what is given to us, not separate aspects of the experience, and certainly not separate sensations that are somehow put together.

The error of the “atomic” view of perception is that its proponents are reifying aspects of a perceptual relationship, its sensory qualities, and treating them as if they were independently existing things. As I mentioned in my summary of Hume’s view, he literally believes that these qualities can exist separately because they can be distinguished, as in 1.4.3.7 of the Treatise. But this is a plain category mistake: Hume and the sensualists are treating qualities as if they were entities, things. Qualities, aspects are not independently existing phenomena, but are aspects of something, whether it is an entity or its derivatives like an action or a relationship. Aristotle was the first to explain this point about the priority of the categories, and Rand recently reminded of this truth in the last century. Perception is composed of neither individual sensation “atoms,” nor of separately existing qualities “bundled together” into things. Perception is a unitary, seamless awareness of things, not the result of a composition of sensations.

Conclusion

From the foregoing, I hope the reader can appreciate how important it is to get one’s foundational views clearly sketched out, and checked against the facts of reality. Both Hobbes and Hume begin as standard empiricists with their advocacy of perception and sensations, but subsequently tear up their own base. Hobbes literally calls the senses “deceivers”; Hume ends up holding that the objects we think that we perceive and the causality we think operates in the world are just figments of our imaginations. Hume has no earthly idea what is causing the perceptions, as he dismisses the idea that experience warrants our ideas of independently existing objects.

Everything that they build up in their philosophies rests on a foundation that even they criticize and do not fully support. Sensualism ended in Humean skepticism, which is what it deserved. Sense perception is not the end-all and be-all of knowledge, contra sensualism. Trying to make it so can only end in disaster. To escape the pitfalls of sensualism, we must put both the perceptual level and the conceptual level on a better foundation than the sensualist school planned for it. The Objectivist axioms and corollaries are exactly the sorts of things needed to explain what can claim to be self-evident and to show the field of philosophy how to properly ground its own systems.

References

Binswanger, Harry. How We Know. New York: TOF Publications, 2014.
      “Cognition.” Oxford Dictionary. Online version.
      http://www.oxforddictionaries.com/definition/english/cognition
Hobbes, Thomas, and W G. P. Smith. Hobbes's Leviathan: Reprinted from the Edition of 1651.
      Oxford: Clarendon Press, 1909. PDF version.
Hobbes, Thomas. The Elements of Law Natural and Politic. Online version. http://www.thomas-
      hobbes.com/works/elements/3.html
Hobbes, Thomas, and Mary W. Calkins. The Metaphysical System of Hobbes: In Twelve
      Chapters from Elements of Philosophy Concerning Body, Together with Briefer Extracts 
      from Human Nature and Leviathan. Chicago, Ill.: Open Court Pub. Co, 1913. Online version.
      https://archive.org/details/cu31924014604007
Hume, David. An Enquiry Concerning Human Understanding. Online version.
      http://www.bartleby.com/37/3/
Hume, David, L. A Selby-Bigge, and P. H Nidditch. A Treatise of Human Nature. Oxford:
      Clarendon Press, 1978. Print.


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