I and Thou is written as a series of long and shorter aphorisms, divided into three sections. The aphorisms within each section are arranged without any linear progression; that is, they are not supposed to be read as subsequent steps in an argument, but as related reflections. Each of the three sections taken as a whole comprises a stage in Buber's larger argument. The first part of the book examines the human condition by exploring the psychology of individual man. Here Buber establishes his crucial first premise: that man has two distinct ways of engaging the world, one of which the modern age entirely ignores. In the second part of the book, Buber examines human life on the societal level. He investigates both society itself and man as he exists within society. In this section, Buber claims that modern society leaves man unfulfilled and alienated because it acknowledges only one of our modes for engaging the world. The third part of the book deals with the subject of religion. Building on the conclusions of the first two sections—that man has two ways of engaging the world, and that modern society leaves man alienated by valuing only the first of these—Buber tells us how to go about building a fulfilling, meaningful society (a true community) by making proper use of the neglected second mode of engaging the world, and by using this mode to relate to God.
The fundamental concept underlying the entire work is the distinction drawn in the first section between the two modes of engaging the world. The first of these, which Buber calls "experience" (the mode of 'I–it'), will be familiar to any reader, since it is the mode that modern man almost exclusively uses. In Experience, man collects data, analyzes it, classifies it, and theorizes about it. The object of experience (the It) is viewed as a thing to be utilized, a thing to be known or put to some purpose. In experience we see our object as a collection of qualities and quantities, as a particular point in space and time. There is a necessary distance between the experiencing I and the experienced It: the one is subject, and the other object. Also, the experiencing I is an objective observer rather than an active participant in this mode of engaging the world.
In addition to this familiar mode of engaging the world, there is also another mode available to us, one which we must necessarily make use of in order to be truly human. In this mode, which he calls "encounter" (the mode of I–You), we enter into a relationship with the object encountered, we participate in something with that object, and both the I and the You are transformed by the relation between them. The You we encounter is encountered in its entirety, not as a sum of its qualities. The You is not encountered as a point in space and time, but, instead, it is encountered as if it were the entire universe, or rather, as if the entire universe somehow existed through the You. We can enter into encounter with any of the objects that we experience; with inanimate objects, with animals, and with man. With man the phenomena of encounter is best described as love. We can also, however, enter into encounter with a being that cannot be the object of experience: God. This type of encounter is the subject of the third section of the book.
In part two, Buber takes the conclusions that he has drawn about man's fundamental psychology—the identification of man's two equally important means of engaging the world—and puts these conclusions to work in sociological reasoning. He looks at modern society and notes how it is entirely built up based on the mode of I–It. Politics, economics, public institutions, even much of personal life, are all fundamentally grounded in the fact that we view every other being as an It, rather than as a You. Modern man has come to feel alienated fundamentally because modern society is exclusively an It-world. Existential angst, worries of meaninglessness, and the sense of impending doom that most modern human beings feel at some point in their life (often in the dead of night, when they cannot sleep) are all the result of our strict reliance on experience to the exclusion of encounter.
In the third section, Buber gives us his solution to modern man's woes. He has already made it clear in the previous two sections that this solution will involve opening ourselves up to encounter and building a society based on relation to You's rather than experience of It's. In section three, he reveals how we should go about doing this. All encounters, he begins by telling us, are fleeting; it is only a matter of time before any You dissolves into an It again and as soon as we begin to reflect on the You it becomes an It. Love, then, is a constant oscillation between encounter and experience, and it does not wholly fulfill our yearning for relation. In every human encounter that we undergo, we feel that there could be something more, something more lasting and more fulfilling. This "more" is encounter with God, or absolute relation. We cannot seek our encounter with God, but can only ready ourselves for it by concentrating both aspects of our self (the I of experience and the I of encounter) in our souls. If we ready ourselves for encounter it will definitely occur, and the proof that it has taken place will be in the transformation that we undergo; after absolute encounter we come to see every other being (nature, animals, people) as a You. We come to feel affection for everyone and everything, and to have a sense of loving responsibility for the whole course of the world. This transformation, Buber tells us, is divine revelation. It is salvation. Filled with loving responsibility, given the ability to say "You" to the world, man is no longer alienated, and does not worry about the meaninglessness of life. He is fulfilled and complete, and will help others to reach this goal as well. He will help to build an ideal society, a real community, which must be made up of people who have also gone through absolute relation, and are therefore willing to say "You" to the entire world.