While no evidence can be conclusive, we seem to be inclined to accept something (whose existence has been conjectured) as actually existing if its existence is corroborated; for example, by the discovery of effects that we would expect to find if it did exist. However, we may say that this corroboration indicates first, that something is there; at least the fact of this corroboration will have to be explained by any future theory. Secondly, the corroboration indicates that the theory that involves the conjectured real entities may be true, or that it may be near to the truth (that it has a good degree of verisimilitude). 
I suggest that the entities which we conjecture to be real should be able to exert a causal effect upon the prima facie real things of an ordinary size: that we can explain changes in the ordinary material world of things by the causal effects of entities conjectured to be real. 
World 3 is the world of the products of the human mind. These products, in the course of evolution, were first probably encoded only in the human brain and even there only in a fleeting way. That is to say, if an early man told a story of a hunt, or something like that, then the story would be both encoded in his brain and in the brains of his listeners, but it would soon be forgotten and in a sense disappear. The more characteristic objects of World 3 are objects which are more lasting. They are, for example, early works of art, cave paintings, decorated instruments, decorated tools, boats, and similar World 1 objects. At that stage there is perhaps not yet a need to postulate a separate World 3. The need arises, however, when it comes to such things as works of literature, theories, problems, and, most clearly of all, such things as, for example, musical compositions. A musical composition has a very strange sort of existence. Certainly it at first exists encoded in the musican’s head, but it will probably not even exist there as a totality, but, rather, as a sequence of efforts or attempts; and whether the composer does or does not retain a total score of the composition in his memory is in a sense not really essential to the question of the existence of the composition once it has been written down. But the written-down encoding is not identical with the composition – say, a symphony. For the symphony is something acoustic and the written-down encoding is obviously merely conventionally and arbitrarily related to the acoustic ideas which this written-down encoding tries to incorporate and to bring into a more stable and lasting form. So here there already arises a problem. Let us pose the problem in the following way. Clearly, Mozart’s Jupiter Symphony is neither the score he wrote, which is only a kind of conventional and arbitrarily coded statement of the symphony; nor is it the sum total of the imagined acoustic experiences Mozart had while writing the symphony. Nor is it any of the performances. Nor is it all performances together, nor the class of all possible performances. This is seen from the fact that performances may be good or less good, but that no performance can really be described as ideal. … In that sense the World 3 object is a real ideal object which exists, but exists nowhere, and whose existence is somehow the potentiality of its being reinterpreted by human minds. [449-50]
There are three points to observe in the attempt to discover the nature of truth, three requisites which any theory must fulfil.
(1) Our theory of truth must be such as to admit of its opposite, falsehood. A good many philosophers have failed adequately to satisfy this condition: they have constructed theories according to which all our thinking ought to have been true, and have then had the greatest difficulty in finding a place for falsehood. In this respect our theory of belief must differ from our theory of acquaintance, since in the case of acquaintance it was not necessary to take account of any opposite.
(2) It seems fairly evident that if there were no beliefs there could be no falsehood, and no truth either, in the sense in which truth is correlative to falsehood. If we imagine a world of mere matter, there would be no room for falsehood in such a world, and although it would contain what may be called ‘facts’, it would not contain any truths, in the sense in which truths are things of the same kind as falsehoods. In fact, truth and falsehood are properties of beliefs and statements: hence a world of mere matter, since it would contain no beliefs or statements, would also contain no truth or falsehood.
(3) But, as against what we have just said, it is to be observed that the truth or falsehood of a belief always depends upon something which lies outside the belief itself. If I believe that Charles I died on the scaffold, I believe truly, not because of any intrinsic quality of my belief, which could be discovered by merely examining the belief, but because of an historical event which happened two and a half centuries ago. If I believe that Charles I died in his bed, I believe falsely: no degree of vividness in my belief, or of care in arriving at it, prevents it from being false, again because of what happened long ago, and not because of any intrinsic property of my belief. Hence, although truth and falsehood are properties of beliefs, they are properties dependent upon the relations of the beliefs to other things, not upon any internal quality of the beliefs. [87-8]
James Boswell relates in his Life of Johnson how he and Dr Johnson were discussing Bishop Berkeley’s solipsistic theory of the non-existence of the material world. Boswell remarked that although no one believed the theory, no one could refute it either. Dr Johnson kicked a large rock and said, as his foot rebounded, ‘I refute it thus.’ Dr Johnson’s point was that Berkeley’s denial of the rock’s existence is incompatible with finding an explanation of the rebound that he himself felt. Solipsism cannot accommodate any explanation of why that experiment – or any other experiment – should have one outcome rather than another. To explain the effect that the rock had on him, Dr Johnson was forced to take a position on the nature of rocks. Were they part of an autonomous external relaity, or were they figments of his imagination? In the latter case he would have to conclude that ‘his imagination’ was itself a vast, complex, autonomous universe. […]
But Dr Johnson’s idea is more than a refutation of solipsism. It also illustrates the criterion for reality that is used in science, namely, if something can kick back, it exists. ‘Kicking back’ here does not necessraily mean that the alleged object is responding to being kicked – to being pysically affected as Dr Johnson’s rock was. It is enough that when we ‘kick’ something, the object affects us in ways that require independent explanation. For example, Galileo had no means of affecting planets, but he could affect the light that came from them. His equivalent of kicking the rock was refracting that light through the lenses of his telescopes and eyes. That light responded by ‘kicking’ his retina back. The way it kicked back allowed him to conclude not only that the light was real, but that the heliocentric planetary motions required to explain the patterns in which the light arrived were also real.
By the way, Dr Johnson did not directly kick the rock either. A person is a mind, not a body. The Dr Johnson who performed the experiment was a mind, and that mind directly ‘kicked’ only some nerves, which transmitted signals to muscles, which propelled his foot towards the rock. Shortly afterwards, Dr Johnson perceived being ‘kicked back’ by the rock, but again only indirectly, after the impact had set up a pressure pattern in his shoe, and then in his skin, and had then led to electrical impulses in his nerves, and so forth. Dr Johnson’s mind, like Galileo’s and everone else’s, ‘kicked’ nerves and was ‘kicked back’ by nerves, and inferred the existence and properties of reality from those interactions alone. What Dr Johnson was entitled to infer about reality depends on how he could best explain what had happened. For example, if the sensation had seemed to depend only on the extension of his leg, and not on external factors, then he would probably have concluded that it was a property of his leg, or of his mind alone. He might have been suffering from a disease which gave him a rebounding sensation whenver he extended his leg in a certain way. But in fact the rebounding depended on what the rock did, such as being in a certain place, which was in turn related to other effects that the rock had, such as being seen, or affecting other people who kicked it. Dr Johnson perceived these effects to be autonomous (independent of himself) and quite complicated. [86-7]
Jede Wirklichkeit impliziert von selbst eine Grenze. Was gar nicht existiert, lässt sich niemals begenzen.
Deshalb besteht eine Affinität, ein Bündnis zwischen dem Totalitarismus und der Lüge.
Vielen Leuten kommt allerdings der Gedanke an eine totale Macht gar nicht nicht in den Sinn; diese Vorstellung würde sie ängstigen. Sie ist schwindelerregend, und es braucht eine Art von Größe, um sie auszuhalten. Wenn solche Leute sich für eine Partei interessieren, begnügen sie sich mit dem Wunsch, sie möge wachsen – doch so, wie etwas wächst, das keine Grenze in sich trägt. Wenn es in diesem Jahr drei Mitgleider mehr gibt als im letzen oder die Spendensammlung hundert Francs mehr eingebracht hat, sind sie zufrieden. Aber sie wünschen, dass dies unbegrenzt immer so weitergeht. Nie würde ihnen einfallen, dass ihre Partei irgendwann zu viele Mitglieder, zu viele Wähler, zu viel Geld haben könnte. [17-8]
Do abstract, non-physical entities exist? Are they part of the fabric of reality? I am not interested here in issues of mere word usage. It is obvious that numbers, the laws of physics, and so on do ‘exist’ in some senses and not in others. The substantive question is this: how are we to understand such entities? Which of them are merely convenient forms of words, referring ultimately only to ordinary, physical reality? Which are merely ephemeral features of our culture? Which are arbitrary, like the rules of a trivial game that we need only look up? And which, if any, can be explained only in a way that attributes an independent existence to them? Things of this last type must be part of the fabric of reality as defined in this book, because one would have to understand them in order to understand everything that is understood. [222-3]