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									       Course Announcements
• Group Take-Home Assignment
  – Group Take-home assigned Tuesday the 19th
     • Will be due by class time Thursday the 28th
     • Grading will be competitive
• Midterm Exam
  – Individual (in-class) on 26th (Tuesday)
  – Group (in-class) on 28th (Thursday)
            Groups for Exam 2
• Group 1

• Group 2

• Group 3

• Group 4

• Group 5
              Today’s Plan
• Announcement about Midterm Exam
  – Group and Individual Component
  – Take-home portion

• Lecture on Ligament & Tendon Notes
  (Lecture 8)
  – Last of midterm material
  From last time…
• Biomechanics of Cartilage
  –   Structure and Function
  –   Loading
  –   Remodeling
  –   Injury and healing
  –   Changes with age
  Biomechanics of
Tendon and Ligament

      ESS 4361-001
       Lecture 8
 Reading: N&F Chapter 4
               Overview
• Composition and Structure
  – Collagen, Elastin and Ground Substance
  – Vascularization
  – Insertion to Bone

• Mechanical Properties
  – Loading
  – Viscoelastic Behavior
• Failure and Injury
• Factors Influencing Biomechanics
                       Joint Structure
• Passive Structures
   – Tendon
   – Ligament
   – Joint Capsule
• Function
   – Ligaments and Joint Capsule
      •   Connect bone and bone
      •   Joint stability
      •   Guide joint motion
      •   Prevent excessive motion
      •   Static restraint
   – Tendons
      •   Attach muscle to bone
      •   Transmit tensile loads from muscle to bone
      •   Produce joint motion
      •   Maintain body posture
      •   With muscle (muscle-tendon unit) acts as a dynamic restraint
      •   Enables muscle belly added distance from joint, thus added mechanical
          advantage
     Composition and Structure
• Dense organized connective tissue
   – Parallel fiber collagen orientation

• Sparse vascularization
• Collagen (Type I)
   – Strength and Flexibility

• Few cells (fibroblasts)
• Abundant cellular matrix
   – 70% water
   – 30% solid
      • Collagen (75%), ground substance, elastin
   – Slightly more collagen in tendon
   – Slightly more elastin in ligament
          Tendon and Ligament
• Principal cells are fibroblasts

• Characteristics
   – Regularly arranged dense fibrous tissue
   – Fibers organized into parallel bundles
   – Primarily contain fibers and extracellular matrix
     components
   – Great tensile strength
   – Able to resist stretch in essentially one direction
     (parallel to fibers)

• Includes tendons, ligaments and aponeuroses
                       Tendons
• Characteristics
  – White, collagenous flexible bands
  – Blood vessels, nerves and lymphatics are evident

• Structure
  – Tropocollagen
     • Basic building block of tendon
  – Mainly Type I collagen
  – Small quantities of elastic fibers

• Function
  – Connect muscle to bone
  – Crimped when relaxed (no tensile load)
                    Ligaments
• Characteristics
  – Dense, regular connective tissue
  – Duller white compared with tendon, slightly more elastic
    and reticular fibers

• Structure (unique)
  – Collagen fibers may be arranged in parallel, obliquely or
    even in a spiral arrangement
  – Specific to individual function

• Function
  – Connect bone to bone
  – Resist tensile forces along the fibers alignment
                  Ligaments

• Name Classification Criteria
  – Attachment (e.g. coracoacromial)
  – Shape (e.g. deltoid)
  – Function (e.g. capsular)
  – Position or Orientation (e.g. collateral, cruciate)
  – Position relative to the joint capsule (e.g.
    extrinsic, intrinsic)
  – Composition (e.g. elastic)
                 Ligaments
• General Make-up
  – Fibroblasts are main cell
  – Proteoglycans also present
    • May play role in mechanical behavior due to large
      percentage of water
                        Ligaments
• Sensory Receptors are available
  –   Ruffini corpuscles
  –   Pacinian corpuscles
  –   Golgi tendon organs
  –   Free-nerve endings
• Provide position, movement and pain information
  – Albeit role in joint proprioception remains unclear
       • Perhaps related to stiffness of muscle controlling joint

• Yellow Elastic Ligaments
  – Less common
  – Parallel elastic fibers surrounded by loose connective
    tissue
  – E.g. vocal cords, ligamenta flava in vertebrae
  – Ligamentum nuchae (in animals)
                            Fascia
• Catch all term
• Characteristics
   – Dense, fibrous, unorganized tissue
   – Collagenous with some elastic and reticular elements

• Structure
   – Interwoven, meshlike, non-parallel fibers

• Location
   – Sheaths around organs, blood vessels, bones and cartilage
   – Dermis of the skin

• Function
   – Provides framework and support for muscles
   – Fibers withstand stretch in many direction due to unorganized
     fiber arrangement
              Fascia and Tendon
      Parallel and Series Elastic Elements
• Fascia runs parallel with contractile elements
   – Enhances stiffness (somewhat) of muscle belly

• Tendon runs in series with contractile elements at
  tendon junction
   – Extend the muscle-tendon length
   – Enhance range of motion and contractile velocity
   – Enhance moment arm of muscle and thus force
     capabilities of muscle (e.g. patellar tendon)

• Many times modeled as springs and viscous
  dampers
                 Vascularization
• Limited vascularization
  – Affects healing process and metabolic activity
• Ligaments
  – Hypovascularized
  – Vascularization originates at bone insertion sites
  – Fatigue
     • Accumulation of damage from normal activities
     • Results from decreased diffusion or hindered vascularization,
       which commonly increases risk for rupture

• Tendons
  – Blood supply direct from perimysium, periosteal insertion,
    and surrounding tissues
  – Vascular vs. Avascular
     • Sheathed tendons and synovial diffusion
     • Implications for tendons compared with ligaments?
                 Outer Structure
• Paratenon
   – Term referring to the loose areolar connective tissue surrounding
     tendon
   – More structured than the same structure that surrounds ligaments
   – Sometimes runs the length of the tendon
   – Sometimes found only where the tendon meets the joint
       • Why?

• Function
   – Protects the tendon
   – Enhances gliding

• Epitenon
   – Synovial layer below the paratenon
   – Facilitates gliding of the tendon
        Ligament Attachment
Insertion to Bone
• Direct
  – Similar to Sharpey fibers

• Indirect
  – Collagen fibers from ligament blend with
    periosteum of bone
           Tendon Attachment
• Insertion to Bone
  – Gradual transition from tendon to fibrocartilage, then
    to subchondral bone (Sharpey’s fibers)


• Insertion to Muscle (Myotendinous Junction)
  – Specialized region at end of tendon, containing multiple
    infoldings (thus increase surface area, reducing stress
    during contraction)
  – Junctions experiencing shear loading are stronger than
    junctions experiencing tensile loads (perpendicular)
                  Aponeurosis
• Fibrous, ribbon-like extension of the tendon
  (flattened tendon)
• Characteristics
  – Similar in composition as tendon
  – Whitish due to collagen
  – Fibers run in a single direction

• Function
  – To increase the attachment site for the muscle
     • Enhance torque capabilities of the muscle by extending
       moment arm of the muscle
 Attachment
   to Bone
1. Parallel
   collagen fibers

2. Unmineralized
   fibrocartilage

3. Mineralized
   fibrocartilage

4. Cortical bone
              Loading Behavior
• Viscoelastic
   – Fluid and solid properties

• Tendon
   – Strong in tension to transfer muscle force to bone
   – Flexible to allow movement at joints

• Ligaments
   – Pliant
   – Flexible to allow natural movements of bones
   – Strong and extensible for suitable resistance to forces
              Biomechanical Properties
•        Tensile Tests
     –     Preferred norm
     –     Tissue loaded in tension
           until rupture
     –     Load (P) is plotted


1.       Toe Region
2.       Secondary or Linear
         Region
3.       End of Linear Region
4.       Maximum Load
     •     Ultimate tensile strength
             Modulus of Elasticity
Young’s Modulus
   E = σ/ε


Young’s Modulus by Region
1. E increases gradually
2. E stabilizes
3. Stiffness increases greatly with additional load
4. Abrupt rupture
                Loading Behavior
• Physiological strain during locomotion
  –   2 to 5% in vivo
  –   Proportional to intensity
  –   Quite a bit less than values reported in testing
  –   Suggests that in vivo, ligaments are loaded to only a
      portion of their ultimate stress

• Viscoelastic Behavior
  – Increased strain rates result in steeper linear portion of
    curve
       • Indicates greater stiffness at higher strain rates
  – Increased stiffness suggests greater stored energy
       • Area under the curve
       • Indicates greater force required to rupture
           Loading Behavior
• Cyclical Loading
  – Stress-strain curve is displaced to the right with
    each cycle
  – Evidence of a non-elastic (plastic) component
    with repeated loading
     • Amount of permanent deformation is progressively
       increased with each deformation
  – Increased elastic stiffness with each cycle
                 Materials Testing
                Cyclical Load Testing
• Stress-Relaxation
  –   Loading is halted below linear region
  –   Strain is held constant over an extended period
  –   Stress initially decreases rapidly at first then slows
  –   Repeated cycles
       • Decrease in stress becomes less pronounced

• Creep
  –   Loading is halted below linear region
  –   Stress is held constant over extended period
  –   Strain increases quickly at first, then slows
  –   Repeated cycles
       • Increase in strain becomes less pronounced
 Stress-
Relaxation
   and
  Creep
Clinical Usefulness of Viscoelasticity
• Casting to treat clubfoot in children
  – Cast creates constant load which results in
    slightly increased strain over time


• Casting to treat toe-walking in children
  – Cast creates constant strain, to which
    eventually the muscle-tendon adapts
  Bone-Ligament-Bone Loading
• ACL Loading Experiment
  – Slow Tensile Loading
     •   Bony insertion proved to be weakest component
     •   Tibial evulsion fracture
     •   Decreased energy storage compared with fast loading rate
     •   Stiffness was comparable

  – Fast Tensile Loading (similar to actual injury rate)
     • Ligament was weakest component (60% of the time)


• As loading rate is increased, bone demonstrates
  greater in strength compared with ligament
  Ligament and Tendon Failure
• In vivo
   – Excess loading results in microfracture
      • Prior to even the yield point (in test conditions)
   – Complete rupture occurs readily
   – Abnormal joint motion
      • Usually results in further damage to surrounding tissues (e.g.
        joint capsule, ligaments, blood vessels)


• Even if the ligament is strained beyond its yield
   – Joint holds together (ligament remains attached, etc.)
Ligament
 Testing
Ligament and Tendon Failure
Ligament and Tendon Failure
ACL Failure
              340-390N Load
Repetitive Injury – Achilles Tendon
 Videos – Clinical Tests
         Knee
          Lachman Test
    Anterior Drawer Test (Test)
Anterior Drawer Symptomatic Large
  Videos – Clinical Tests
      Knee & Ankle
Posterior Cruciate Ligament Testing
       Posterior Drawer Test
     Achilles Tendon Tear Test
        Videos – Clinical Tests
              Shoulder
        Anterior Drawer Test Shoulder
       Anterior Drawer for the Shoulder
Posterior Instability shoulder load and shift test
    Factors Affecting Function
•   Aging
•   Pregnancy
•   Mobilization and immobilization
•   Diabetes
•   Anti-inflammatory drugs
•   Hemodialysis
•   Graft selection
  Factors Influencing Properties
• Maturation and Aging
  – Up to 20 years
     • Increase in # and quality of collagen cross links
     • Increase in diameter of collagen cross links (variable 20-180nm)
     • Increased strength
  – 20-60 years, and 60+
     • Increased fibril concentration (i.e., more), but…
     • Marked and consistent decrease in diameter (less variable 110
       to 120nm)
     • Decreased water content
     • Collagen strength plateu…followed by a decrease in tensile
       strength and stiffness

• Pregnancy and Postpartum Period
  – Increased laxity in tendons and ligaments in pubic area
  – Tensile strength decrease
  – Stiffness decreases, but is restored later
    Factors Influencing Properties
• Mobilization and Immobilization
   – Tissues remodel in response to loading, thus stronger and stiffer
   – Tissues become weaker in response to reduced loads
       • Example: ACL - >30% decrease in load to failure and energy stored to
         failure following 8 week casting
   – Implications?

• Diabetes Mellitus
   – Loss of ability to oxidize carbs, and excessive urine excretion
   – Higher rates of tendon contracture, tenosynovitis, joint stiffness, and
     capsulitis
   – Related (causative) to osteoporosis
   – Ligament/Tendon Properties
       • Elastic properties remain similar, however increased viscous response in
         diabetics
       • Lower stiffness, no difference in strength
       • Method of failure differs due to weaker bones in diabetics (i.e., bone
         fractures)
  Factors Influencing Properties
• Steroid Use
   – Corticosteroids (long-acting) inhibit collagen synthesis
   – Decreased ligament stiffness, failure load, and energy absorption
     (with extended use)
   – Semi-regular injection (e.g. every 3 days, for a month)
       • Increase in tensile strength and max load stiffness of muscle tendon
       • Decrease in strength of the bone attachments of ligaments

• Estrogen receptors in ACL
   – Physiological levels of estrogen reduce collagen production by 40%
   – Pharmacological levels of estrogen reduce collagen production by
     50%
   – Estrogen fluctuations may alter ligament metabolism, thus altering
     composition, increasing likelihood of injury
  Factors Influencing Properties
• Nonsteroidal Anti-inflammatory Drug Use
  (NSAIDs) – Short Term Use
  – Aspirin, acetaminophen, indomethacin
  – Increase tensile strength
  – Increase in proportion of insoluble collagen, total
    collagen and likely cross-linkages in collagen

• Hemodialysis
  – Tendon failure associated with renal failure does occur
    (36%)
  – Hyperlaxity in ligaments and tendons common,
    resulting in joint hypermobility
   Factors Influencing Properties
• Grafts
  – Reconstruction
  – Allografts versus Autografts
  – Harvests: Bone-Patellar Tendon-Bone, Achilles Tendon,
    Hamstring Tendon, Patellar Tendon
  – Allografts
     • Years after reconstruction did not resemble normal tendon grafts or
       ACL fibers
  – Autografts (patellar tendon)
     • Considerable changes within 2 year period
     • Appearance of normal ligament tissue
     • Mechanically not the same as ACL, but will provide mechanical
       restraint allowing functional knee motion
     • Streching will affect stiffness, but not necessarily strength of the graft
                 Summary
• Composition and Structure
  – Collagen, Elastin and Ground Substance
  – Vascularization
  – Insertion to Bone

• Mechanical Properties
  – Loading
  – Viscoelastic Behavior
• Failure and Injury
• Factors Influencing Biomechanical Properties
  For Next Time…


• Midterm Assignment
  – Group Take-home assigned Tuesday the 19th
     • Will be due Thursday the 28th
     • Grading will be competitive
• Midterm Exam
  – Individual (in-class) on 26th (Tuesday)
  – Group (in-class) on 28th (Thursday)
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 any one particular colour.’
                      Colour
• It is the same basic form of argument as with the
  table: different points of view will yield different
  appearances
• So what legitimacy do we have to select any
  appearance as THE one.
               A Critique
• I am tempted to ‘go to town’ on Russell
  here but that may just confuse matters;
  however, allow me to present one critique
• The very idea of immediate, non-inferential
  knowledge and/or experience
• Remember I asked the question ‘If the
  world was all one colour would we have the
  notion of or experience of colour?’ No.
                  A Critique
• If you grasp what that question asks of you, you
  will then have a powerful argument against
  Russell’s theory of sense datum as that which is
  immediately, atomistically and singularly grasped

• Further class example: ‘see the difference’

• Moving On
           Matter and Idealism
• ‘The real table, if it exists, we will call a ‘physical
  object’. Thus we have to consider the relation of
  sense-data to physical objects. The collection of
  all physical objects is called ‘matter’.
• Berkeley – ‘to be is to be perceived’
• However, such idealists do agree that our sense
  data comes from an external source
• What can we say of this external source?
                    Doubts
• (1) is there a real world world which we naturally
  believe in causing our sense data
• (2) Could it all be strictly internal – a dream or
  hallucination
• (3) could there be other people and how do we
  know that they are possessors of mind
• (3) is especially problematic considering that all
  that we can know about people is derived from
  observing their bodies
                     Doubts
• Of the self: ‘But the real Self is as hard to arrive
  at as the real table and does not seem to have the
  absolute, convincing certainty that belongs to
  particular experiences……So far as immediate
  certainty goes it might be that the something
  which sees the brown colour is quite momentary,
  and not the same as the something which has some
  different experience the next moment’
• Does this make sense?
                     Certain
• ‘Thus it is our particular thoughts and feelings that
  have primitive certainty’
• How do we come to believe in ‘public neutral
  objects’ underlying the sense data:
• ‘Thus it is the fact that different people have
  similar sense-data, and that one person in a given
  place at different times has similar sense-data ,
  which makes us suppose that over and above the
  sense-data there is a permanent public object
  which underlies or causes the sense-data…’
      Belief and the Real World
• What’s wrong with this argument?
• So how do we argue that there is a real world, the
  world as we perceive it?
• It is logically possible that the world could be
  utterly different from the way we perceive it and
  yet cause the perceptions we do have: ‘But
  although this is not logically impossible, there is
  no reason whatever to suppose that it is true; and it
  is in fact a less simple hypothesis…’
       Belief and the Real World
•   We argue to the best explanation
•   An illustration: the cat
•   What best accounts for our sense data of the cat?
•   Answer: there is a real cat
•   If the cat was just sense data it wouldn’t exist
    when we are not having the sense data but this
    leaves characteristics of our experience quite
    mysterious – i.e., why the cat is hungry between
    meals when we do observe it
      Belief and the Real World
• If we extend such reasoning to all of our
  experiences we shall come to the conclusion that
  ‘every principle of simplicity urges us to adopt the
  natural view, that there really are objects other
  than ourselves and our sense data…
• Not how we naturally come to this belief – such a
  belief is instinctive without needing the help of
  philosophy
      Belief and the Real World
• Even this argument for the best explanation rests
  on instinctive beliefs since ‘all knowledge, we
  find, must be built up upon our instinctive beliefs’

• So then what is the job of the philosopher
        The Job of Philosophy
• ‘It should take care to show that, in the form in
  which they are finally set forth, our instinctive
  beliefs do not clash, but form a harmonious
  system. There can never be any reason for
  rejecting one instinctive belief except that it
  clashes with others; thus, if they are found to
  harmonize, the whole system becomes worthy of
  acceptance’
• Do we expect more from reason?
     The Job of the Philosopher
• A humble modest role for the philosopher: a
  philosopher is not an oracle nor can she claim to
  be the possessor of secret mysteries
• But a worthy task nevertheless
   Chapter Three: The nature of
             Matter
• The question: ‘What is the nature of of this real
  table, which persists independently of my
  perception of it?(50)
• Science’s answer: all natural phenomena ought to
  be reduced to motions
• Light, Heat, sound – wave motions, i.e., Heat =
  mean molecular kinetic energy – the mean kinetic
  energy of the molecules in this room is about 6.2 x
  10-21 joules
   Chapter Three: The nature of
             Matter
• i.e., oscillatory frequency of sound – say a ‘C’ one
  octave above middle C – is 527 hz; colour – a
  certain wavelength triplet of electromagnetic
  reflectance efficiencies
• The problem – is this our experience?
• What is the relationship between our sense data
  and real physical objects?
         With respect to Space
• ‘The real space is public, the apparent space is
  private to the percipient. In different people’s
  private spaces the same object seems to have
  different shapes; thus, the real space, in which it
  has its real shape, must be different from the
  private spaces. The space of science, therefore,
  though connected with the spaces we see and feel,
  is not identical with them, and the manner of its
  connexion requires investigation’
        So what can we know?
• ‘We can know only what is required in order to
  secure the correspondence’
• Our knowledge of space is restricted to that which
  can account for our sense data experience – i.e.,
  relations of distances
• Similar comments about time – i.e., ‘time drags’
  ‘time flies when you are having fun’
            Thus we conclude
• ‘Thus we find that, although the relations of
  physical objects have all sorts of knowable
  properties, derived from their correspondence with
  the relations of sense-data, the physical objects
  themselves remain unknown in their intrinsic
  nature, so far at least as can be discovered by
  means of the senses’ (53)
             We can conclude
• We should take Russell strictly here
• ‘We can know only what is required to secure the
  correspondence’
• What does this mean?
• What kind of limitation of knowledge does it
  impose?
• Why, in considering this type of limitation, could
  Russell be justified in the claim that ‘the physical
  objects themselves remain unknown in their
  intrinsic nature’?
        Chapter Four: Idealism
• Could what we understand by ‘physical objects’
  be ideas or something mental?
• Idealism: ‘We shall understand by it the doctrine
  that whatever can be known to exist, must be in
  some sense mental’
• The target: Berkeley – ‘to be is to be perceived’,
  all of our experiences are, contrary to appearance,
  of ideas, minds experiencing such ideas, other
  minds and God
        Where Berkeley is right
• Sense data subjective – they ‘depend upon us as
  much as upon the tree, and would not exist if the
  tree were not being perceived’ (52)
• If sense data = reality then, according to Russell,
  Berkeley would be convincing; ‘however, ‘it is
  necessary to prove, generally, that by being
  known, things are shown to be mental’(54)
           Berkeley’s Fallacy
• Berkeley’s idealism rests on a conceptual mistake
• A failure to clearly distinguish between a mental
  act of awareness or apprehension and the object of
  the awareness itself
• The act = mental; the thing, the object itself, the
  mental object – can we conclude that it is mental?
            Berkeley’s Fallacy
• Berkeley equivocates on the term ‘idea’ :
  ‘Berkeley’s view, that obviously the colour must
  be in the mind, seems to depend for its plausibility
  upon confusing the thing apprehended with the act
  of apprehension.’ (55)
Why I am not convinced by this.
• Earlier of the idealists Russell states: ‘They
  almost all agree that, however much our sense-
  data…may depend upon us, yet their occurrence is
  a sign of something existing independently of us,
  something differing perhaps, completely from our
  sense-data whenever we are in a suitable relation
  to the real table’ (47)
 Why I am not convinced by this
• In this chapter, of Berkeley he states: ‘All our
  perceptions…consist in a partial participation in God’s
  perceptions, and it is because of this participation that
  different people see more or less the same tree’ (54)
• Furthermore, of Berkeley he states: ‘He admits that there
  must be something which continues to exist when we go
  out of the room or shut our eyes, and that what we call
  seeing the table does really give us reason for believing in
  something which persists even when we are not seeing
  it…as an idea in the mind of God’ (46)
       Why I am not convinced
• Keeping these quotes in mind, if Berkeley did not
  grasp a distinction between mental act and mental
  object he would not (a) appeal to an external
  source for our ideas and (b) would hold the
  doctrine that things do go out of existence when
  we no longer perceive them.
       Why I am not convinced
• The fact that Berkeley maintains and understands
  the constancy of a thing/idea means that he has
  grasped the distinction between an awareness of
  an idea (our awareness which is not constant) and
  its nature (which is) or external source, even if he
  hasn’t thematized this as ‘mental act’ and ‘mental
  object’.
• Of course if we believe that objects have to be
  material then this must be Berkeley’s fallacy – but
  that’s just begging the question
           Russell partly right
• The real hang up for the Berkeleyian idealists is as
  Russell states: ‘..he thinks that this something
  cannot be radically different in nature from what
  we see…’(46)
• The hang up – if it is one – for the idealists is an
  ontological principle: only ideas can stand in
  relation to ideas. It is utterly mysterious how
  matter could be translated, converted, into an idea
• In a sense agreeing with the problem of
  ‘interactionism’ leveled at Cartesian Dualism
     Another Idealist Challenge
• ‘It is often said, as though it were a self-
  evident truism, that we cannot know that
  anything exists which we do not know”
• Need to distinguish what we mean by
  ‘know’ here. For….
     Two Kinds of Knowledge
• Knowledge by acquaintance: the kind you have
  when you are aware of a sense datum (i.e., see a
  patch of red)
• Knowledge by description: a true judgment, when
  we judge that a certain proposition is true
• We need to develop this distinction in greater
  detail
     A Preliminary Clarification
• There are things we can know without being
  acquainted with – i.e., the emperor of China, all
  past historical figures
• There are also things that we can know without
  having not had any acquaintance with
• ‘But it is not true that, conversely, whenever I can
  know that a thing of a certain sort exists, I or some
  one else must be acquainted with the thing. What
  happens, in cases where I have true judgement
  without acquaintance, is that the thing is….
     A Preliminary Clarification
• …is known to me by description, and that, in
  virtue of some general principle, the existence of a
  thing answering to this description can be inferred
  from the existence with which I am acquainted.’
  (56)
• Think here of the theoretical entities that science
  often postulates whose only justification is internal
  to the theory itself – they are not observable
  themselves. However, if they are useful in
  explaining what we do observe, what we are…
    A Preliminary Clarification
• …acquainted with, we can be ‘realists’ with
  respect to them.
• Questions: (1) Russell is an empiricist
  philosopher. In what way does he resemble
  Hume?
• (2) In what manner is he different from
  Hume?

								
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