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As we all know, Kant wrestled with Cartesian skepticism for a long time. And of course, Descartes himself did, but he appeals to a version of the ontological argument which is not very persuasive.

What other philosophers tried to give a serious refutation of skepticism about the external world that in some way goes beyond an appeal to common-sense (which at a first glance is the basis for George Edward Moore’s “here is one hand”-argument)?

  • Radical skepticism is acknowledged to be irrefutable, a sufficiently determined skeptic can deny any premise used to "refute" him. The arguments (including Moore's) are rebuttals to the skeptical arguments. Dretske noticed a common closure pattern used in them:"if you know that this is a chair then you know that you are not dreaming/hallucinating/deceived, you can not know that, ergo" and argued that the closure is invalid, see Epistemic Operators. General approach in formal epistemology is to isolate the epistemic assumptions that skeptics require. – Conifold Aug 29 at 20:23
  • @Conifold imho the Cartesian skeptic is seductively convincing because he manages to drive a wedge between the objective and the subjective world. Especially since with the cogito he can present something “irrefutable” – with virtually no disagreement. He isn’t just a skeptic naysayer, he gives a standard that he expects proponents of the external world to meet. – wolf-revo-cats Aug 29 at 20:46
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    @PeterJ So we should all accept that it is possible that a conscious being is manipulable up to the level of making their whole life an illusion. It sounds like a joke to me, a very dangerous one. – Pyrott Aug 31 at 13:37
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    This is just a side note, but Descartes can't really be said to have "wrestled with Cartesian skepticism". Descartes was never worried about refuting skeptical hypotheses. "Skepticism" there is not a threat, not an opposing position, but a tool, a method. And the method is purgative: the Meditations are meant to help the individual expose and rid himself of his habit of relying on sense-perception and imagination as a source of knowledge. The meditator knowingly and willingly enters into the suspension and makes use of the method and puts the method down when he is done meditating for that day – transitionsynthesis Aug 31 at 20:15
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    @RiccardoAquilanti - It's not a joke, it's the Perennial philosophy. No manipulation is required. There would be a sense in which life is an illusion but it might be less misleading to say it is not what we think it is. Scepticism in Western thought is a dead-end that looks like the end of philosophy but this is only because it rejects the form of scepticism endorsed by mysticism. This is an optimistic doctrine that explains why we cannot falsify solipsism.or prove naive realism. To argue for naive realism is to argue against this doctrine. – PeterJ Sep 5 at 12:56
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Thomistic realism refutes skepticism

This is no place to give you a systematic treatment of the Thomistic view on knowledge, but if you want to defeat poisonous errors of skepticism about the external world you should learn about the Thomistic theory of knowledge. It defeats every form of idealism.

Here is a good introductory article: How Thomistic Realism Refutes Radical Skepticsm. This article just scratches the surface, but it seems good for introduction.

I will present just a sketch of how you might defend realism.

  1. Just as the first object of hearing is sound, so the first object which intellect perceives is being (= that which is); after that, we need to discover following about the concept of being (ie. first Thomistic thesis):

Potency and Act divide being in such a way that whatever is, is either pure act, or of necessity it is composed of potency and act as primary and intrinsic principles

For good treatment of act and potency see: Reality: A Synthesis of Thomistic Thought chapter 5: On act and potency. Actual being is basicly what is now actual, or what is now obtained, and potential being is what can obtain (but one should be careful not to confuse possible being with potential being).

  1. Then you need to learn about the principle of causality which can be stated as follows (this is from Summa):

But nothing can be reduced from potentiality to actuality, except by something in a state of actuality.

When some potential is actualized there needs to exist some actual being that actualizes the potential. It can be shown that one who denies this principle is really denying the principle of non-contradiction. But principle of non-contradiction can not be negated.

  1. Then you need to understand that intellect is all things potentially.

Intellect by its very nature, when it acquires knowledge of something, becomes that thing. But since intellect is potency, when it becomes something actual, or when knowledge occurs, there must be some actual being which is the reason for that actualization. Hence, realism. All this is pretty complicated, for a good introduction to the nature of knowledge see Foundations of Thomistic philosophy chapter 2 on Being and Knowledge by A. G Sertillanges.

Read also Methodical Realism: A Handbook for Beginning Realists for an interesting approach to realism; if you do not want to read the whole thing, you can see just chapter 5 which gives 30 important points about realism.

All this is, of course, an oversimplified picture. But if you want knowledge, you will need to put the effort in.

Conclusion: Propositions like: "radical skepticism is acknowledged to be irrefutable..." or "there is no solution to perpetual how do I know that" or "everything is maybe just in my mind" are errors. These errors poisson the mind (they are however very sophisticated errors which are not trivially solvable) and are refuted by Thomistic Realism. But if you want to overcome errors mentioned, you will need to do a lot of reading and thinking.

  • @thom- Well done, stand your ground. Spinoza accused Descartes of hypocrisy in claiming to doubt anything, especially to doubt anything by conscious choice. Although it is a bit difficult to master, Spinoza's argument claims that once we know something with any degree of certainty, the way our mind's work makes doubting what we know untenable. That is except as some type of rhetorical game. If you think about what type of mind does a person have who claims to know nothing about the world? Not the type, I might venture, that anyone should own up to in a public forum. Sapere Aude. CMS – Charles M Saunders Aug 30 at 22:52
  • "It defeats every form of idealism." - I'm a bit skeptical of this claim. ;) – J D Sep 19 at 14:19
  • @JD Maybe, in reality, you are not reading this comment but rather playing the guitar in the middle of the Amazonia xD, but in case you really do, I am open to respond to objections :) – Thom Sep 19 at 19:44
  • I already made it. I think you may have missed it. – J D Sep 20 at 15:01
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If you are the only being in the world, then what makes your life hard? It is quite reasonable for there to be another part of yourself that hates you. But then, what is the distinction between that other part of yourself and another being? If there is just more of you, at odds with itself, how do you consider yourself to be a single being with a single agenda. But if you are a single being with multiple agenda, then what does that mean?

Ken Wilber, a trans-Jungian theorist who integrates Eastern and Western philosophical traditions into an "integrationist" therapeutic model, points out that these distinctions between single and multiple beings, single and multiple agenda are each sometimes true and sometimes false.

At different levels of aggregation and abstraction, we are all one organism, or we are different species of organisms, or we are different societies of organisms, or we are different families of organisms, or we are simply different individual organisms, or we are a body and a mind, or we are multiple minds despite our identified physical boundary, or we are multiple minds working across multiple identifiable physical boundaries, or... Identity, and with it various related processes of homeostasis, goes all the way up at least to the planet, and all the way down to the folded proteins that power the multiple different kinds of organelles within each cell.

Daniel Dennett is a bit obsessed with the notion that the separate cells of your body were evolved to survive independently and to compete with one another, but they are bound together against their potentially divergent wills to participate in your overall conscious process. This is parallel to the way microbiology points out that the organelles that exist in our cytoplasm, which make individual cells work, like mitochondria and Golgi bodies, originate as separate prokaryotic species with very divergent agendas, and are enslaved in a way by the nucleus as a resident overlord.

So it seems silly to answer the question. It can only limit our insight. And it is almost certain to be wrong at least some of the time. If this is all me, it is still all here, in competition and collaboration with itself. If it is not all me, there is a greater me of which it is all part, but there are also sources of agenda outside me. Identifying with different layers of it is productive at different times. That makes what is 'external' a matter of circumstance, and renders this a meaningless question.

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