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Sensation, Perception, and the Problem of the External World Philosophical Background

What is Perception?

stuff in

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What is Perception?

stuff in the world

percepts

Process for: • Extracting information via senses • Forming internal representations of the world

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Plato’s Allegory of the Cave (375 BC)

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Skepticism (the Problem of the External World)

• Meditations on First Philosophy (Descartes, 1641) – undertakes a program of “radical ” to discard any idea that can be doubted • Conclusions – senses can be fooled – all sense data could be caused by “evil demon” – only thing one can be certain of is that he exists • cogito ergo sum (“I think, therefore I am”) – arrives at “dualist” theory of (more on this later)

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Skepticism (the Problem of the External World)

• “Veil of perception” – Nothing is ever directly present to the mind in perception except perceptual appearances, and these can be mistaken or misleading

• How can we ever be justified in our perceptual beliefs? – Any individual perceptual appearance may be of the veridical or the misleading kind • How can we know which is which? – Without good reason for thinking that perceptual appearances are veridical, we are not justified in our beliefs

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Skepticism: Modern Variants

“” • Updated version of “evil demon” thought experiment by Gilbert Harman (1973)

(from https://www.sinfest.net/view.php?date=2006-06-14)

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Skepticism: Modern Variants

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Skepticism: Modern Variants

“What is real? How do you define real? If you’re talking about what you can feel, what you can smell, what you can taste and see, then real is simply electrical signals interpreted by your brain. This is the world that you know

—Morpheus in (1999)

(film clip)

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Reactions to Skepticism

• Metaphysics/ontology (theory of reality) – What is the nature of reality?

• (theory of knowledge) – What/how can we know about reality?

• Philosophy of mind/perception – What is the relationship between perception and “the world”?

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Metaphysics/Ontology (Theory of Reality)

• Question A: Does the external world exist?

– Answer 1: Realism • The external world (i.e., objects, events, etc.) exists independently of our minds • It is sensible to talk about “objective/empirical reality” or “ground truth”

– Answer 2: Non-realism • There is no (fixed) external world. Objects, events, etc. do not exist independently of our minds • It is not sensible to talk about “objective reality” or “ground truth”

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Metaphysics/Ontology (Theory of Reality)

• Question B: What kind of stuff exists in the world?

– Answer 1: Dualism (Descartes’ answer) • There are two kinds of stuff • Usually “mind/soul” and “matter/body”

– Answer 2: Monism • There is only one kind of stuff • Idealism: there is only mental stuff (Berkeley’s answer) – Our perceptions of external objects are just ideas • Materialism: there is only physical stuff (Hobbes & most modern scientists) – Only physical matter and energy exist – The mind is what the brain does – Methodological vs. ontological materialism

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Epistemology (Theory of Knowledge)

• Question A: How do we obtain knowledge – Answer 1: (a posteriori knowledge) • We learn, by induction and through experience, by observing the world through our senses • Any induction is fallible, so we only know things probabilistically; nothing is certain • “Truth” is determined by correspondence/pragmatics – i.e., do our beliefs predict our observations? • Our minds start as a blank slate (Locke, “tabula rasa”)

– Answer 2: /Nativism (a priori knowledge) • True knowledge is innate. We “learn” deductively through reason • “Truth” is determined by coherence – i.e., is our belief system internally consistent?

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Philosophy of Mind

Q: What is the relationship between “things in the world” and “representations in our heads”?

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Answer 1: Naïve Realism (aka “common sense realism”)

• We perceive the world as it exists • Our minds have direct access to reality

external reality

exact map of reality

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Answer 2: Idealism

• The only reality is that of mind/ideas • We have no evidence of an external (physical) world

? reality

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Answer 2: Idealism

Descartes’ (skeptical) supposition • Also: • Plato’s allegory of the cave • Brain in a vat • The Matrix

“evil demon” reality

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Answer 2: Idealism Bishop George Berkeley (1685-1753) - idealist, empiricist - all objects of perception are mental objects

God reality

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Answer 2: Idealism Solipsism • The only reality is that of my mind/ideas • I have no evidence of an external world, nor of other minds

X reality

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Answer 3: Representative (indirect) realism

• We perceive the external world indirectly & imperfectly via intermediary sense data representation of reality external reality

sense data

processing

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Philosophical Stances Assumed in this Course

• Metaphysical: realism, materialism – Why? Because science only really makes sense under these assumptions • Epistemology: focus on empiricism (but with acknowledgement of innate knowledge) – Why? We’re studying perception, which is necessarily empirical, as is science – Both extremes (blank slateism vs. extreme nativism) are silly – Note that you can view much innate knowledge as a form of empirical learning (via natural selection) on a geological timescale • Philosophy of mind/perception: representational realism – Why not idealism? Because science doesn’t make sense under this assumption – Why not naïve realism? Argument from illusion

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Argument from Illusion

• The fact that we are sometimes mistaken in our sensory perceptions indicates that we do not directly perceive the world • Naïve realism is false

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Hermann Grid

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Simultaneous Contrast Illusion

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Understanding Perception

In this course, “understanding” perception has a meaning analogous to “reverse engineering”. For example, to “understand” how a perceptual system accomplishes a task means that:

1. We can write down an algorithm (recipe) for accomplishing that task • In principle, we could use it to design a robot to perform the same task

2. We know where and how the algorithm is implemented in the nervous system

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Example:

What is this?

This is the input to your brain

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“Solving” perception in this case would mean building an algorithm that (using just the values below) could answer any question about the image that we can

For example: • Who is this? • What’s his facial expression? • What is he wearing?

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A Simpler Example: Cochlear Implants

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