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What definition of to exist, of to be, and of real preponderates contemporary philosophy? How do the terms differ from one another?

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Some suggestions for research: (1) existence, most generally, belongs to the intersection of metaphysics and logic. Pioneering works in this domain include Shane's Quine suggestion and [Carnap 1950]. Start there and proceed until you reach Williamson's Modal Logic as Metaphyiscs, a contemporary masterpiece of this genre. (2) being I know nothing about, but Heidegger, Husserl, Bolzano, Towardowski, etc. do, so check them out. (3) if reality means physical reality, then an (unhelpful) definition is: whatever the natural sciences say exists. Philosophers of science might be helpful there. – Hunan Rostomyan Mar 29 '14 at 20:08

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Most philosophers would hold that all three of those terms are synonymous. The seminal paper on this question is Quine's On What There Is. For Quine the only things that exist are physical objects.

However, this wasn't the view of some very important ancient philosophers like Aristotle and there are at least a very few contemporary philosophers who would hold that different kinds of thing exist in different ways. One of these folks is Kris McDaniels, who has a paper called Ways of Being or something like that.

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In 'The Basic Problems of Phenomenology' Heidegger describes two types of being: the existence of things perceived is one type, and such things are extant beings; but there is also the existence of the perceiver, which is a qualitatively different type of being, (transcendental). Heidegger called this the ontological difference.

The quotes below are from Basic Problems, on the terminology of 'real' in Kant :-

The concept of reality and the real in Kant does not have the meaning most often intended nowadays when we speak of the reality of the external world or of epistemological realism. Reality is not equivalent to actuality, existence, or extantness. It is not identical with existence, although Kant indeed uses the concept "objective reality" identically with existence.

The Kantian meaning of the term "reality" is the one that is appropriate to the literal sense of the word. In one place Kant translates "reality" very fittingly by "thingness,” "thing-determinateness." The real is what pertains to the res. When Kant talks about the omnitudo realitatis, the totality of all realities, he means not the whole of all beings actually extant but, just the reverse, the whole of all possible thing-determinations, the whole of all thing-contents or real-contents, essences, possible things. Accordingly, realitas is synonymous with Leibniz' term possibilitas, possibility. (Page 34) ...

The Kantian concept of objective reality, which is identical with actuality, must be distinguished from the concept of reality as thus elucidated. The realness or being-something that is fulfilled in the object thought in it, in its Objekt, is called objective reality (objektive realität]. That is to say, it is the reality exhibited in the experienced entity as an actual existent entity. In reference to objective reality and reality in general, Kant says: "As regards reality, we obviously cannot think it in concreto without calling experience to our aid. For reality can only relate to sensation as material of experience and is not concerned with the form of the relationship, whereas, if we so chose, this form could be made subject to a play of fictions." Kant here separates objective reality as actuality from possibility. (Page 37)

I find this give a nice conception of reality from the existential point of view, because one is dealing with possibilities. Scheodinger's cat is alive and dead in 'reality', until the actuality is ascertained.

Other philosophers no doubt use different terminology.

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Heidegger's ontological difference is not between the object and the perceiver nor between their existence. The difference is between the entity and the fact it is. It applies well to both the physical object and human being. I always think of this as the difference between life and living being. We have the verb to live. We can create noun life and living being. But living being is not the same as life. We can say many things about living beings that are not true about life, like living beings live but life does not live. The same is here, beings (entities) "are" but being "is not". – Zefiryn Mar 31 '14 at 21:06

I don't know if this would really answer your question, but it should at least indicate where & what to look for :)

to be - in general form means the same as to exist (a special example as expressed by Descartes: Cogito ergo sum - popularly translated as 'I think therefor I am (have body)' <=> because 'something' needs to contain or do the thinking)

real - is the same as to exist. (To say something is real if it exists is tautologous.)

to exist: This becomes valid if and only if there is someone/thing that can be aware, otherwise the question would not exist(!). In other words it refers to perceptibility: what you are aware of or can infer from what you are aware of (including the phenomenon of awareness).

This changes your question to become: "What is awareness?"

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They differ to the extent that the author using them fails to properly specify exactly what they are talking about. The issue of what these three terms actually refers to, I think is answered nicely thus:

According to the article series 'Highly Advanced Epistemology 101 for Beginners' by Eliezer Yudkowsky; we can recursively define the physical universe.

His work takes root in the idea that all complex systems can be understood by examining their constituent parts and applying the theory of emergent phenomena: Brains are collections of neurons, neurons are collections of organic molecules, organic molecules are collections of atoms, atoms are collections of subatomic particles, etc.

Then, he introduces the concept that mathematics is a separate thing from the computer or human reasoning about it; and the mathematical reasoning is in some sense a "window" into the world of maths. This is akin to platonism but subtly different: He describes maths as (paraphrased) "The fact that when you start out with a certain well-defined symbol string, and apply certain well defined transformations to it, you always end up with the same result." No mystic dimension, just a statement of an apparent fact about the universe.

He then goes on to argue that in fact, what happens when one reasons about mathematics, is that some physical system takes on the properties of a mathematical model of an axiomatized theory. Pebbles in bowls behave like Peano Arithmetic (with an upper bound), as do our neurons when we count, sum and multiply, and as do the transistors in a computer when we use it for calculation.

His central thesis is then that everything can be split up into parts which can be put in either "Physical universe" or "Mathematical object" boxes.

The definition he provides of the "Physical universe" is based on Judea Pearl's Causality (2000), which defines Causality in terms of conditional probabilities. That there is a great web of causal arrows, connecting all events in the physical universe, going from past to future events, to a greater or lesser extent.

Yudkowsky's definition of the Physical Universe is then that one starts with the phsyical process that implements one's own cognition (the brain), adding it to the set of "the physical universe". Then for every object in the "physical unvierse" one adds all objects causally influencing or being causally influence by said objects.

That is, everything ever influenced by or influencing anything influencing or being influenced by you, in any in principle observationally determinable way is 'physical' and 'exists in the real world'. Everything else is mathematics.

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Oh God, that sounds terrible. I would expect nothing better from [Less] Wrong. Most imediately, this account seems to fail on account of the "fundamental problem of causal inference" (Statisics and Causal Inference, Holland). I'm a big fan of Pearl, but its far from complete as a theory, there are many problems with it as a model of causal inference, and that doesn't even touch on the problems with using it to talk about causality in a sense that is consistent with how physicists see things. – Lucas Mar 29 '14 at 16:39
@Lucas Argumentum ad absurdium et hominem aside; Yudkowsky does a bang-up job at materialist philosophy, because it is a by-product of MIRI's work on General AI. Their work is very in lieu of Quine and Kahnemann. – Karl Damgaard Asmussen Mar 30 '14 at 15:11
Being part of a group of nutters does not recommend him to me (nor did reading his work!). I don't know Quine's work well enough to comment, but the reason Kahnemann was so influential is because he took assumptions about rationality and tested them empirically. He showed some fundamental cases where the models failed which lead to a whole field of empirical research. In this sense I see him as the opposite of Less Wrong and MIRI who take for granted the very same principles that Kahnemann addressed critically. – Lucas Mar 30 '14 at 18:27
@Lucas I get the feeling you are not actually familiar with the Lesswrong community's actual beliefs, in the sense of "know your enemy." I will agree that Eliezer Yudkowsky's public work in the Sequences 2006-2009 is mostly didactive, as is the one I reference in my answer. His work on GAI however is rock-solid mathematical epistemology. The primary criticism of classical philosophy posed by Luke Muehlhauser (Ex.Direc of MIRI) Philosophy: A Diseased Discipline which a question like this is a clear instance of, I try to remedy. – Karl Damgaard Asmussen Mar 31 '14 at 15:07
Well that article didn't update my beliefs about them in any new direction. It does allow me to contextualise my previous comment, LM says "AI is useful because it keeps you honest: you can't write confused concepts or non-natural hypotheses in a programming language." whereas Kahnemann took the opposite approach, along the lines of "We have this model which is formally correct, does it actually apply - no" - in other words, he showed that a "confused concept" had indeed been "[written] in a programming language". Their unwavering faith in formal models really shows with attitudes to Pearl. – Lucas Mar 31 '14 at 15:55

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