"Perhaps this book will be understood only by someone who has himself already had the thoughts that are expressed in it--or at least similar thoughts.--So it is not a textbook.--Its purpose would be achieved if it gave pleasure to one person who read and understood it. The book deals with the problems of philosophy, and shows, I believe, that the reason why these problems are posed is that the logic of our language is misunderstood. The whole sense of the book might be summed up in the following words: what can be said at all can be said clearly, and what we cannot talk about we must pass over in silence. Thus the aim of the book is to draw a limit to thought, or rather--not to thought, but to the expression of thoughts: for in order to be able to draw a limit to thought, we should have to find both sides of the limit thinkable (i.e. we should have to be able to think what cannot be thought). It will therefore only be in language that the limit can be drawn, and what lies on the other side of the limit will simply be nonsense. I do not wish to judge how far my efforts coincide with those of other philosophers. Indeed, what I have written here makes no claim to novelty in detail, and the reason why I give no sources is that it is a matter of indifference to me whether the thoughts that I have had have been anticipated by someone else."
A key statement in this preface is that he is seeking to draw a limit to thought. That does not mean to create restrictions for thinking. Instead Wittgenstein is setting out to show that that by mapping the possibilities and impossibilities of thought, we can describe the limits of reality. After all, if we cannot think it then it cannot be -- in our world at least. That is, for something to exist in the world (in actuality or imagination), it must be potentially thinkable by us, otherwise it could never register on our
minds at all.
Now, someone might reply; "Well but what if there are things in existence that are beyond our human ability to imagine or conceive?" That seems like a fair objection, though now we must ask the obvious question; "What do you have in mind? Can you give an example?" The answer to that must be "No." If it is not thinkable, then no one can describe it or exemplify it in anyway. So, while the objection seems as if it refers to something, it really has no referent at all. This is what Wittgenstein calls nonsense. No because it is silly, but because a sentence has a sense just in so far as it refers to some possible state of affairs in the world. Even if it is false or ridiculous, a sentence has sense if it refers to a possible combination of elements. This is part of Wittgenstein's explanation of what meaning is.
As already noted, the words;
there are things in existence that are beyond our human ability to imagine or conceive.
seem like an ordinary sentence, so we are likely to take it seriously (i.e. grant that it could be true). Yet, Wittgenstein's analysis shows that such a string of words has no referent (does not refer to anything) and so is without a sense -- is senseless -- is nonsense -- is meaningless. Such a sentence is neither true not false. Because only sentences with a sense can be true or false (the sense is what truth and falsity is based on).
This sort of thinking can make many people's heads swim -- you sort of get it but cannot quite pin it down. That is an effect that comes from running up against the limits of language -- these are not limits that we create, they are the limits of what language can do. Wittgenstein depicts a model of the human mind such that "the limits of my language mean the limits of my world." Thus, through the philosophical study of language, we can identify the boundaries of philosophical thought.