Episode Sixteen – Optical Illusions, Proof of the Validity of the Senses

Today we are going to discuss the validity of the senses and how optical illusions prove that the senses are valid.

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Episode Transcript

[Editorial: Please note that this may not exactly match the audio. However, there should be no significant differences. There is also some background thumping in some of the later minutes of the audio. We were unable to remove this. Sorry if this bothers you, but please tolerate it if possible.

The title uses the word “proof”. Note that this is used in the colloquial sense of “an attempt to convince someone of”. The validity of the senses is not really subject to proof. Instead, all proof relies on the validity of the senses.

A better title might have been something like “Demonstration of the Validity of the Senses”.]


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Hi everyone! This is episode sixteen of the Metaphysics of Physics podcast and I am Ashna, your host and guide through the hallowed halls of the philosophy of science. Thanks for tuning in!

Today we are going to discuss the validity of the senses and how optical illusions prove that the senses are valid.

Modern philosophers claim that the senses distort the data provided to them. And that as a result, you cannot fully trust your senses.  We shall analyse what the senses are, how they operate, and why this claim is false.

What Are The Senses?

The senses take in inputs and according to their mode of operation, cause the experience of sensations.  For example, human eyesight works by absorbing light, which leads to the sensations of sight.  An aspect of the sensations of sight is colour.

However, dogs and many other animals perceive reality different to humans and other beings that perceive colour. They see the world around them in black, white and shades of grey.

Apes color perception.
Many other apes have tri-chromatic colour perception that humans do.

[Editorial correction: Actually the thing about dogs seeing in black and white is a myth, which we neglected to correct ourselves on. It is now thought that dogs can see in other colors. Granted, a more limited range of colors than humans. But, for the sake of argument and not having to correct the entire article, let us pretend it is true.]

Does this mean that dogs do not see things as they are?  No, their senses are not deceiving them.  Sensory organs are passive; they simply interact with light, sound or other forms of input, respond and send signals. This triggers the last stage of perception, the experience of sensations.

Sensory organs have no power to deceive or to distort their input. They simply receive input and send signals to the brain.  There is no mechanism which they possess which can alter the input. Or by which they can alter the sensations experienced.

Different sensory organs work in different ways, reacting differently to light, sound, touch and so forth.  However, regardless of how your sensory organs react to stimuli, regardless of whatever sensations the organism experiences, reality remains the same.

Your eyes react differently to different wavelengths of light, causing the brain to experience sensations which include various colours.

Dogs are blind to colour, but this only means that their eyes and brain do not react to different wavelengths of light in the same way the eyes and brains of humans and other colour-perceiving organisms do.

What is it that you call “colour”? Is the observed colour in the object? Or is colour a matter of sensation? What do you observe when you experience colour?  You are observing that an entity has properties such that it either emits or reflects light of a certain wavelength.

Are you for instance observing that a blue object has “blueness”?  No, the object does not possess “blueness”. It has such properties that it emits or reflects light of a certain wavelength that causes the sensation “blue” when it reaches our eyes.

Colour is not in the object. Colour is a sensation which is dependent on the visual mode of perception.  Visible objects possess properties which cause the sensations of colour.

Multicolored sky
If the sky has “blueness”, it would be harder to explain why it can be so many different colors at once …

Every organism capable of sensing the world around them has sensory organs that trigger sensations.  The nature of those sensations depends on the nature of the entities observed and how they interact with the sensory organs.

Ah, but a dog sees the world in black and white, and humans see it in colour! Therefore, the vision of a dog is subjective and so is ours.  After all, we know that the world is not black and white, but a world of colours.

Or do we? Perhaps neither are objectively true! Perhaps colour is just in our mind!

These kinds of irrational ideas show why it is important to identify that colour is not in the object, but the result of the means of operation of the sensory organs of certain organisms.

[Editorial: These kinds of irrational ideas also show some of the dangerous conclusions you can reach once you start questioning the validity of the senses.]

The sky has no “blueness”, nor does chocolate have “brownness”. The sky has properties which cause it to scatter light in such a way that, when observed by sensory organisms that respond to different wavelengths of light, cause the organism to experience the sensation of the colour blue.

Chocolate interacts with light in such a manner as to cause such organisms to experience the sensation of some shade of brown.

No matter how hard you look, you will never find “colour” in any object, only properties of the object which cause the object to emit or to reflect light of a certain wavelength.

Sensations are not in the object. The nature of sensations depends on the nature of the entities and how they interact with the sensory organs.

Objects do not possess the innate attributes of “blueness”, “bad odour” or “loudness”.  They possess properties that cause organisms to experience particular sensations when observed by those organisms.

You are receiving sensations based on how your sensory organs operate and the nature of the relevant entities you are observing.

In the case of eye damage such that you experience blurry sensations of sight, your eyes are not distorting reality. You simply experience sensations consistent with your eyes not being able to process light rays properly.

The facts of reality remain constant. Regardless of the facts, your eyes are not able to react to light in the same manner as someone without eye problems.

The fact that sensations can take a different form, depending on the state of operation of the sensory organs, does not mean that the senses are potential agents of distortion.  While they operate at all, sensory organs will trigger whatever sensations their current condition is capable of triggering.

Different organisms might experience different sensations if their sensory organs operate differently.  The fact of differing sensations is not a valid reason to doubt the validity of senses.

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