Buber is extremely conscious of the role that language plays in forming our experience, and therefore begins I and Thou by identifying what he calls the two "basic words" of human language. These basic words are, in fact, word pairs rather than single words. The first basic word is "I–It," while the second is "I–You" (the "I and Thou" of the book's title). In calling these words basic, Buber means to claim that their very utterance establishes a mode of existence; when label someone or something as "It," we become a certain kind of I, which exists in a certain way; when we label something as "You," we become a different sort of I, and exist in a different sort of way. The claim that there are these two modes of engaging with the world around us is the cornerstone of Buber's project. The rest of the work is an attempt to elucidate these two modes of engaging with the world, show us that we have been ignoring the mode of I–You with grave consequences, and instruct us in how to improve our human condition by opening ourselves up to this neglected mode of engaging the world.
The first mode, the mode of I–It, is the mode that will be familiar to all modern readers. Buber calls this mode of engaging the world "experience". In experience, the I acts as objective observer toward the It rather than as active participant in any relationship with the It. The activities of experience are the activities that we associate with thought, both scientific and everyday: observing, cataloguing, calculating, analyzing, describing. The I views the It as an object to be known, manipulated, and utilized. The It appears to the I as the sum of its qualities, as a point in space and time.
Experience is crucial to our survival as human beings. By experiencing the world we come to grasp the order, stability, laws, processes, and systems which we then use for our various purposes. It is through experience that we can come to know the truth, and it is through experience that we acquire a sense of authority and agency in the world. Experience allows us to master the world around us.
As important as experience is, it is not as important as modern man seems to believe. Experience may be necessary for human survival, but it is not sufficient. Modern man acts as if experience were the only means available to him for engaging the world, but there is also the mode of I–You, the mode of encounter. A human being is not fully human, Buber warns, unless he also opens himself up to the mode of I–You, and begins to relate with, rather than master, the world around him.
Reading Buber can sometimes be a frustrating experience because his style of writing is purposely vague and obscure. In an attempt to steer us away from the analytic thought processes of experience, Buber writes in poetic, mystical, aphoristic language. Happily, Buber's obscure assertions can usually understood with a little bit of patience (though sometime says outright that his statements are obscure because the notion he is trying to convey cannot be truly captured in words).
When trying to understand the notion of basic words, it is important to realize that when Buber claims that what we "say" determines our mode of engaging with the world, he does not literally mean that the sounds which come out of our mouth determine our mode of engaging with the world. You could make the sound "you" to someone or something and still be experiencing, rather than encountering, that object. What is relevant is not what you say with your mouth, but what you say with your "being"; that is, how you approach your object, how you view it. You can say "you" all you want, but if you are viewing your object as the sum of their qualities, then you are experiencing him or her.
The really puzzling thing about Buber's basic words is that they do not seem to be words at all, but, rather, sentence fragments. "I" is a word, "It" is a word, but "I–It does not sound like a word. Consider another possible word, "cat-fat". We have a word "cat" because there are objects out in the world that we conveniently group together under that heading. We have a word "fat" because there are other objects that we conveniently group together with that adjective. We do not have a word "cat-fat" because we do not need one. We can put together the word "cat" with the word "fat", and thus pick out all fat cats. Similarly, you might think, we do not need a word "I–It" because we have the word "I" and we have the word "It" and all we need to do in order to pick out the mode of "I–It" is to put these words together. "I–It" does not even seem like a word, then, much less like of the two most basic words of all language.
Once you see why this is odd, you have gone a long way toward understanding Buber's fundamental claim. Buber's point here is precisely that "I–It" and "I–You" are not formed by putting together the single words "I", "It" and "You"; in fact, there is no word "I" at all (as Buber puts it, "there is no I as such"); there is only the I that is part of "I–It" and the I that is part of "I–You". This is why these basic words are so basic: they determine our very way of existing. We cannot exist, cannot be an "I", outside of one of the modes which are picked out by these words.
Turning now from basic words to the mode of experience, it is important to remember that experience does not only refer to what we might call "scientific reasoning". Inner emotions can be the object of experience just as well as sensory observations can, and we can have experience with regard to mysterious or supernatural subjects just as easily as we can have experience of the laws of physics. What makes something experience is not the content (e.g. personal emotion vs. natural phenomena; angels vs. plutonium), but the attitude. As long as the goal is to get information, to know the It, or to see what use the It can be used for, what is taking place is experience.