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Picasso's senses were right when he painted the ugliness of this philosophical error.
(Pablo Picasso. Lee Miller. Source)
We regularly hear some people claim that "Our senses deceive us!"
Should we be worried about this? Yes, absolutely! To understand why, we can take the metaphor of metrology, and the "standard kilogram" at the Bureau international des poids et mesures in Paris. If a little prankster fiddled with the standard kilogram in Paris, and nobody noticed, it would be all of Science, and even world trade which would suffer disastrous consequences!
It's a bit the same thing in Philosophy. There is nothing in our intellect which wasn't, at some point of time, in our senses. If our senses deceive us, truth becomes an illusion, our thoughts are silly mind-games, and Philosophy is nothing more than a quest for inexistent answers to meaningless questions. And if we destroy Philosophy, we destroy by the same token Ethics (the fourth part of Philosophy), therefore good and evil don't exist anymore!
Before accusing our senses, we should at least try to understand them. Our senses are somewhat like measuring instruments. But we know that we must respect some guidelines when using measuring instruments:
2.1) One single measuring instrument can't measure everything. What would you think about a person who complained that his bathroom scale was unable to measure the number of decibels pumped out by the loudspeakers of his sound system? That would be ridiculous! A bathroom scale doesn't measure sound levels, but body mass!
2.2) Even concerning what it can measure, an instrument can't "cover" all possible values. If you try to weigh a single raisin on your bathroom scale, it will probably tell you "0.0 kilograms", even if a raisin obviously weighs something. And if you go outside early in the morning in your dressing gown, to put your bathroom scale in front of the big garbage truck just as it's going down your street, then the result will probably be also just as disappointing! A measuring instrument (as far as we know) always has a range of values, a minimum and a maximum that must be respected, otherwise the device will be destroyed, or at least it won't give correct results.
2.3) A measuring instrument has limited precision. This rule is a bit like rule #2.2 above. Not only is an instrument unable to measure all possible values, but it can't measure infinitely small variations either. That's why good measuring instruments have labels saying something like: "Precise at more or less this value, between this minimum and maximum of it's normal operating range".
2.4) Even concerning what it can measure, in the right range, with the proper precision, an instrument must be in good working order. After a big garbage truck has gone over it, a bathroom scale might still be in working order, but then it might not. If you weigh yourself after this adventure, and your scale tells you you've lost 400 kilograms in a single day, you mustn't believe it too quickly!
Our five senses are not bathroom scales or voltmeters, so they don't measure kilograms or volts. For the past two thousands years or so (because of the ubiquitous Aristotle), we divide in three classes what our senses perceive:
3.1) Proper sensible. For the eyes, it's color and light intensity. For ears, it's sound. For the taste, it's sweet, or sour, etc.
3.2) Common sensible. These are things like the number, the extent, etc. If you look at fireworks for example, your eyes might see three flashes, your ears might hear three "kabooms", and your young child might squeeze your hand very strongly three times, etc.
3.3) Sensible by accident. These are things like the nature of the physical things which surround you. When you arrive in your kitchen in the morning and you see your dear wife, in fact, strictly speaking, you don't see your wife but a patch of colors with a certain extent.
Our senses don't deceive us, concerning their proper sensible, when they are used within the range of permitted values, with the appropriate precision, and the sense-organ is not sick or damaged.
I can't demonstrate here this thesis of Criteriology (first part of Metaphysics, itself the third part of Philosophy). It would be too long and requires skills I don't have. Nevertheless, excellent authors can do this for you (like for example Aristotle, or Saint Thomas Aquinas, and I hope someday the OSThoPhiT). On the other hand, I can look at some typical arguments given by people who claim that "our senses deceive us".
One of the more frequent lies is the story about the star which is seen by the eye, even though the star has ceased to exist millions of years ago. In fact, the eye is not deceiving us; it really perceives true photons which hit the retina (proper sensible). It's our reason which incorrectly infers the current existence of that star (sensible by accident). This is even more forgivable given that our senses are used almost always for distances where light travels for a negligible amount of time before hitting our retinas. Normally, cavemen didn't chase woolly mammoths located millions of light-years away!
Another typical lie concerns the whole series of optical illusions. Of course, it's quite strange to take optical illusions as a "proof" that our senses deceive us, since we carefully catalog them in books called "optical illusions"! If our senses deceived us, we wouldn't even know they are illusions!
Optical illusions are generally explainable either by #2.3 and 2.4, or #3.3 above. For example, when we go to the movies, we think we're looking at actors who move, but in fact they are still pictures. It's just that the human eye can't perceive a movement which is too fast (#2.4), so the succession of still images (about 36 per second) seems to be like one single image that moves. Coming back to our bathroom scale, if you were able to remove yourself from your bathroom scale, replace yourself with a raisin, then get back up on your scale in less than 0.0000001 seconds, your bathroom scale would probably claim that the raisin is really, really heavy!
The other category of optical illusions generally occurs when we attribute to the eye judgments which in fact are made by our reason (sensible by accident), like the relative length of two lines, or a picture which looks like a dog to some people, and an old grandmother to others, etc.
There are many other such attacks on the reliability of our senses, which can be easily explained somewhat along the same lines as what we've just done.
If our five senses hired a lawyer, quite probably the people who claim that "our senses deceive us" would be sued for slander! Our senses never signed a contract saying they would never be sick, or that they could perceive everything under any circumstances.
If we respect our senses, and avoid asking them more than what they can tell us, they will be immensely more reliable than the bad skeptical and relativist philosophers.
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