Hell this thread got me thinking. Studied the idealist philosopher Berkeley in AS Philosophy and decided to grab some arguments from Bertrand Russell's A History of Western Philosophy (which is brilliant BTW) on the subject of how the mind perceives the world and whether we can get any knowledge from it. For any unfamiliar with the language 'percepts' describes supposed knowledge of the world which the mind 'discovers' through perceiving particular events and logically deduce the premises of why the event happened - we feel we know these things to be 'real'.The first two arguments are those of idealists, who believe that our perception of reality is entirely composed in the mind, not from the world in which the perceiver feels immersed. I will follow with a short summary in brackets of the arguments in somewhat more simplified terms.
"1. We may deny totally the validity of all inferences from my present percepts and memories to other events. This view must be taken by any one who confines inference to deduction. Any event, and any group of events, is logically capable of standing alone, and therefore no group of events affords demonstrative proof of the existence of other events. If, therefore, we confine inference to deduction, the known world is confined to those events in our own biography that we perceive--or have perceived, if memory is admitted."
(We cannot use our recollection of events to prove that the knowledge we deduce from them is true, as our recollection is subjective and the knowledge might only apply to that one given event as we experience it. Simply paraphrased; just because when a tree fell before in your presence it made a sound, does not mean that should the same thing happen again but with no one to witness it, a sound would be produced - just because you perceived it once or even 100 times doesn't prove it would happen again.) The laws of probability makes this claim somewhat untenable.
"2. The second position, which is solipsism as ordinarily understood, allows some inference from my percepts, but only to other events in my own biography. Take, for example, the view that, at any moment in waking life, there are sensible objects that we do not notice. We see many things without saying to ourselves that we see them; at least, so it seems. Keeping the eyes fixed in an environment in which we perceive no movement, we can notice various things in succession, and we feel persuaded that they were visible before we noticed them; but before we noticed them they were not data for theory of knowledge. This degree of inference from what we observe is made unreflectingly by everybody, even by those who most wish to avoid an undue extension of our knowledge beyond experience."
(Relatively self-explanatory - while we might think we know objects are in a room with us, perhaps we saw them when we came in, when our focus is elsewhere, we can infer that those objects are still in the room with us from our recollection alone - but until we actually perceive them we don't know that they are still present.) Common-sense realism would deny this position, if you trust in it.
"3. The third position--which seems to be held, for instance, by Eddington--is that it is possible to make inferences to other events analogous to those in our own experience, and that, therefore, we have a right to believe that there are, for instance, colours seen by other people but not by ourselves, toothaches felt by other people, Pleasures enjoyed and pains endured by other people, and so on, but that we have no right to infer events experienced by no one and not forming part of any "mind." This view may be defended on the ground that all inference to events which lie outside my observation is by analogy, and that events which no one experiences are not sufficiently analogous to my data to warrant analogical inferences."
(We can conceive of a colour-blind person seeing a completely different world to ours, even though we (not colour-blind) can't ourselves perceive those colours normally because perhaps; in a previous event, we perceived of something that others did not (a percept nonetheless). However, should someone claim they can see the distant future with extreme accuracy, we cannot compare that with a similar experience to our own (no one can truly perceive the future before it becomes the present moment - aside from anticipating set events which still might change at any time) and thus we cannot infer any truth from it as it does not compare to our normal perception.)
"4. The fourth position is that of common sense and traditional physics, according to which there are, in addition to my own experiences and other people's, also events which no one experiences--for example, the furniture of my bedroom when I am asleep and it is pitch dark. G. E. Moore once accused idealists of holding that trains only have wheels while they are in stations, on the ground that passengers cannot see the wheels while they remain in the train. Common sense refuses to believe that the wheels suddenly spring into being whenever you look, but do not bother to exist when no one is inspecting them. When this point of view is scientific, it bases the inference to unperceived events on causal laws."
(The argument which seems to make the most sense, but still requires us to accept the world as it is to our senses. Casual laws are scientifically measurable, and so we feel we can trust that their effects will reflect in the world even if we don't perceive it i.e. when a tree falls we know that kinetic energy forms waves of air pressure that will make a sound - even if no one can hear it. But of course if you can accept any of the elements of the first two arguments - this might not be provable.)
ANYHOW.. I hope this has illuminated the discussion with some logical analysis. Looking into Berkeley and other idealists like Hegel is a real brain twister but its fascinating to hear these guys arguing against our most core beliefs about the world around us. I have a lot of respect for free thinkers.
TLDR; Perception works, so solipsism is difficult to justify.