Well, Descartes claims that since we know we are sometimes deceived by our senses, we might always be deceived by our senses. And he gives some examples of sense-deception. You might ask how he knows that we are sometimes deceived by our senses, if he thinks we never know when we are not. It is not merely our senses that deceive us, but they also can correct us when we are deceived by them. Otherwise, we could not know we have been deceived in the first place.

Thanks for that. I had a similar idea floating round but for some unknown reason it would not allow me to articulate it.

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Its another tree falling another musing what if and buts of philosophy ...I ventured into this forum through boredom.. interest..I had no real idea what philosophers actually did but im a bit disappointed they do much the same things as other mortals but with education..

What is 'sense deception'?
Like that the light and colors that we perceive exist not 'out there', as perceived, but in mind as concept? That 'out there' is absolutely dark?
That what you mean?

No. But we may think we see a cat, but since it is dark, what we really see is an old shoe. Like that.
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You'd better go to the movies. Philosophers are mortals. But I am flattered to hear you thought we were gods.

One line of argument could be that Descartes does not really account for how these sense impressions are "taken into" the mind; this is often a problem with dualists who argue that the mind is one kind of substance and the body (e.g. sensed impressions) is entirely another and different.

Nor does Descartes account for our ability to determine if these impressions are deceiving and then go about finding out the true matter (is the tower round or square? Let's go look closer at it). Granted we may be deceived, but are we in each case?

I imagine that Descartes would reply that these attempts at discovering whether deception took place are, themselves, suspect, because they are perceptions too. According to Descartes we can never be absolutely certain that what we believe we observe we observe. We cannot rule out error. And that is, I think, true. Our beliefs about the world are fallible because we, human beings, are fallible creatures. But now we can ask whether absolute, infallible certainty must be our goal. Human knowledge cannot be infallible knowledge. But that doesn't mean we do not sometimes know the truth. Rather, it means that we cannot know we know (be certain) the truth. And that is different. Descartes, I think, confuses knowledge with absolute certainty.

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Kook,

I guess by Descartes theory of sense perception, you are referring to the first meditation up until the cogito argument at the beginning of meditation 2.

If you are making a presentation against it, you cannot mention the theory of sense deception, or more precisely, the argument for universal doubt, without underlining the emphasis Descartes has on Rationalism. The rationalist perspective was in most part championed by Descartes initially, although you could argue that others like Aquinas had an indirect influence on the notion. Also, Descartes had other follow who, though not in tune with his own philosophy in particular, maintained a rationalist perspective like Spinoza and Leibniz. Descartes was very much against the common school of thought at the time, scholasticism, which was in many respects drawn of Platonic and Aristotelian texts rediscovered earlier.

But Descartes is mainly a rationalist. He wants to prove that there is a type of knowledge and that this knowledge comes before the sense. Basically, it's like saying that you, at the moment of your birth, possess, had all the ideas and concepts of the world imprinted in your mind and gradually you became aware of those a-priori (before the senses) pieces of knowledge. Opposed to this view are the Empiricist, such as Locke, Hume and Berkeley. Locke for example had a huge argument at the beginning of his Essay Concerning Human Understanding against innate ideas. These innate ideas are the accumulation of the rationalist perspective, which, as previously discussed, was about knowledge gained before the senses.

So when we come to the first meditation and try to formulate an argument against universal doubt (theory of sense deception), you could argue the problematic occurrences in the rationalist mind frame. In what way does Descartes prove that knowledge is gained before the senses? You could say that when Descartes gets to the second meditation and puts for the argument that, though I (Descartes) can be deceived in almost everything, I cannot be deceived by the fact that I am being deceived. Therefore, I am a thing think (Res Cogitans)., he has over looked a central problem? his own analysis of discernables. I am not sure what copy of Meditations you have, but look up Paragraph 8, Mediation 1.

In that specific section, Descartes has mentioned that "the consideration of composite things are doubtful," there is in fact a matter of un-doubtful nature in "arithmetic, geometry" etc. This is a very implicative statement that, in my opinion, excludes some things from a theory on universal doubt. Think about it? how can you have a theory of universal doubt when you at the same time acknowledge some manner of finite discernable?

What about deliberate sense deception as in cultivating positive thinking. We are told that we must trick our senses into believing things are better than there are until they are. Is that the same thing or not?