Ernst Mach asserted the the world consists entirely of sensations. Does this make him a solipsist, and how might one refute him?

Mach was a 'neutral monist', which means that he held that the fundamental units of reality are neither mental nor physical, but when they are combined in some ways they form minds and when they combined in other ways they form physical objects. He was not a solipsist, since his position allows for the existence of many minds, and perhaps even for the knowledge of their existence. By the way, even if you can't refute solipsists, it is some consolation that you can be certain that if they exist they are mistaken.

I have been reading Bertrand Russell's History of Western Philosophy , and am puzzled by a paragraph in a section on Plato ('Knowledge and Perception in Plato') pertaining to the use of the verb 'to exist'. The paragraph reads as follows: "Suppose you say to a child 'lions exist, but unicorns don't'; you can prove your taking him to the zoo and saying 'look, that's a lion'. You will not...add 'and you can see that that exists'...if you do then you are uttering nonsense. To say 'lions exist' means 'there are lions', i.e. 'x is a lion' is true for a suitable x'. But we cannot say of the suitable x that it 'exists'; we can only apply this verb to a description, complete or incomplete. 'Lion' is an incomplete description, because it applies to many objects: 'the largest lion in the zoo' is complete, because it applies to only one object". What puzzles me about this paragraph is quite how it is, as Russell sees it, nonsensical to say 'there is a lion, and it exists'. Is it because we do not...

Russell may be making the claim that existence is not a property. An individual may have the property of being furry, of making loud sounds, and of living in Regent's Park Zoo, but it does not also have a property of existence. Rather to exist is for those properties to be instantiated. To say that something exists is to say that there is something with various properties, but existing is not one of them.

Does not Descartes beg the question when he argues "I think therefore I exist?" My problem with Descartes' argument arises from his attempt to treat "existence" as a predicate that can be applied to subjects. When he says "I think", the word "I" will have a referent if and only if I exist. So, if the proposition "I think" is meaningful -that is, if it succeeds in attributing the property of thinking to a subject "I"-, it is trivial that I exist. However, in order for the proposition "I think" to be meaningful, I must exist in the first place. So, Descartes seems to beg the question of "my" existence. One might just as well assert, "I dance the funky chicken therefore I exist" or my favorite "I outgrabe therefore I exist" (a reference to Lewis Carroll). Thanks...

I'm not sure that the sentence 'I exist' would be meaningless rather than just false if there were no referent for 'I', but the worry about question begging remains. And you are in good company with the funky chicken. Thomas Hobbes, in his objections to Descartes, asks why 'I think therefore I am' is any better than 'I walk therefore I am' ('Ambulo ergo sum'). One reply Descartes can make to Hobbes is that although existence follows equally from walking and thinking, Descartes has a certainty about his own thought that he does not have about his own walking (since he doesn't even at this stage know he has legs). There does seem to be something particularly secure about the belief that one is thinking -- how could one be mistaken? Indeed the belief is self-verifying. To believe one is thinking makes it so, since belief is thought. Believing one is walking, or dancing, does not make it so, sincd believing is not dancing. Still, I think the worry about question begging remains. For...

Has there every been anything which refutes Descartes' theory that all we can be sure of is that we are thinking things? Is there any proof that we can be sure that other people exist?

It does seem possible that nobody else exists: I just have a very lively imagination. An interesting line of thought to the contrary is that my very ability to think contentful thoughts depends on the existence of other people. In that case, if I can have the contentful doubt whether other people exist, then they do. But can I prove that I have contentful thoughts?

Socrates said, "All I know is that I know nothing". What I'm trying to figure out is this: if I know NOTHING, how do I KNOW that I know nothing? It just goes round in circles thus becoming nothing more than a paradox. Would you agree?

I agree that the sentence "All I know is that I know nothing" is paradoxical, or anyway false, since if there is one thing that you know than you can't know that you know nothing since that isn't true. But we can probably avoid the problem by saying instead "All I know is that I know nothing else".

According to Descartes' demon hypothesis, would it be possible for the demon to deceive us about the rules of logical inference e.g. could my belief in the law of non-contradiction be caused by the demon?

I'm no Descartes scholar and Jay may well be right that actually Descartes held that God makes the laws of logic true, or neccessarily true. But the answer to the question still seems to be that, for all Descartes knows in the First Meditation (before he has convinced himself of the existence of God), he could be wrong even about those laws, and that would be so even if the laws of logic were beyond the control of God or demon.

I think that once Descartes goes beyond the dream to the demon, we could be wrong about anything. It's not that the demon could change the laws of logic: according to Descartes, I believe, not even God could do that. But the demon could make the simplest logical truths seem false to us and the most blatant logical falsehoods seem true. This is what he calls 'hyperbolic doubt', a beautiful expression for a nasty situation. What is mysterious is how he thinks that even 'cogito ergo sum' can survive this kind of warp-drive skepticism.

René Descartes said that "I think therefore I am". Would it not be more true to say: "I am therefore I think"?

I can see why you suggest that "I am therefore I think" is a better way of putting things: existence is necessary for thinking in a way that thinking is not necessary for existing. Indeed existing is necessary for a thing to have any properties whatever, whereas there are things that exist but do not think. But Descartes was here not interested in the order of reality ; instead, as Sean points out, he was interested in the order of knowledge . And he comes to know that he exists by means of his awareness of his thinking, not vice versa .

In the beginning of The Republic , Socrates demonstrates to Thrasymachus, I think, that his theory of justice, i.e., "do good to one's friends and evil to one's enemies", is false because it may be that one has evil friends and good enemies, or be mistaken about in fact who is our friend and who is our enemy. I wonder, though, about this: We are faced with three potential questions. One possible question is "who are our true friends and our true enemies?". Another possible question is "are our true friends good and our true enemies evil?". A third possible question is "what is justice, considered apart from irrelevancies like our friends?". It seems to me that we are much more likely to be right in our judgments about the first two questions than we are in our third. We might be wrong in all three, of course, but if asked to either 1) accurately identify one's friends and evaluate their worthiness or 2) create a theory of justice, I would suggest that the vast majority of people (perhaps why we...

I'm no Plato expert, but I think the main issue here is not how we judge or know what is just, but rather what justice is. If it is not just to do good to an evil friend, this shows that justice isn't the same as doing good to a friend. And it shows this no matter how easy it is for us to identify our friends and to judge their worth.

Hello experts, I have a question that burns my mind. Rorty is usually considered as one of the most significant philosophers of today. However, I simply cannot understand him. Firstly, he speaks against epistemology, yet he argues for "pragmatism" (which I presume is a theory of knowledge). Secondly, he argues against metaphysics, yet he argues for "eliminative materialism" (which I presume is a theory of metaphysics). What is happening here? Seems illogical to me.

Here is a cartoon version of Rorty's radical philosophy. The job of language and thought is not to represent an independent reality, but to give us tools that help us to thrive. Some thoughts and ways of talking are better than others, because more conducive to thriving than others, and Rorty is trying to convince us that pragmatism talk and eliminative materialist talk is better in that sense.

Since Hume clearly says that even children know truths about the unexamined, why do so many intelligent people take Hume to be skeptical of, as opposed to curious about the logic of (justified), inductive practice? I mean, he says, "as a philosopher who has some share of curiosity, I will not say skepticism. I want to learn the foundation of this [inductive] inference." So what's the deal?

As I read Hume, he is saying that children form beliefs about the unexamined, and they do this because of their 'natural instinct' of supposing that the future will be like the past. And Hume thinks that adults are just the same. We might think that we have good reasons for our beliefs about the unexamined, but what Hume's brilliant skeptical argument seems to show is that there can be no such reasons. The deal is that this is, for many of us, a deeply disturbing conclusion.