Buber finds a place for religion outside of rationality in the mode of encounter. He believes that throughout the scientific age the critics of religion have shown correctly that God cannot really fit within the world so long as we are trying to get at the world in the typical way. That is, he recognizes that science and reason can never get us to God, because "it is not as if God could be inferred from anything" (III.4).
God cannot be inferred from anything because the world is causally closed: we never have to appeal to anything outside of the physical world in order to explain a physical phenomenon. All explanations for physical events and states can be given in the form of other physical events and states. Thus, we can never find God through experience, for within the realm of experience we come to know things only by gathering sensory data, and analyzing this data with our reason.
It is not irreligious to claim that the physical world is causally closed (after all, this is certainly the most perfect sort of order that God could have imposed on the world) but if we cannot get at God the way we get at everything else (through reasoning from the data), where can the justification for believing in God's existence possibly come from? Buber says that it comes through encounter. In this mode of engaging we do not gather sense data to be analyzed with reason, rather we simply enter into a relationship with the whole being of whatever it is we are relating to.
This key concept of encounter is one of those notions which Buber tells us can never be made entirely explicit through language. The whole point of encounter is that it cannot be analyzed, described, or reduced down to qualities in space and time. Naturally, this makes it very difficult for Buber to convey the subtleties of the concept to his readers. What does it mean, for instance, to say that the I views the You as more than the sum of its qualities, or in its full being? What specifically is this "more" that we are seeing? Buber cannot tell us, because any aspect of the You that could be described would have to be those qualities that we latch onto in experience. The "something more", by its very nature, cannot be described or analyzed.
The same trouble arises for other aspects of the account: what does it mean to say that in encounter we view the entire Universe through the You? Again, we cannot have more than a vague sense of what this might mean, because Buber cannot really describe encounter to us; we must go through it ourselves in order to know what it is like.
But these difficulties should not make us despair of coming to an understanding of the mode of encounter. As we will see in the next section, much can be gained by comparing encounter to the state of being in love, and other questions can be answered with a little patience and guesswork. For instance, Buber says that we are changed by encounter, and this naturally leads to questions about the nature of this change. Are we changed permanently or only so long as the encounter lasts? Are we spiritually changed, or emotionally, or physically, or mentally? In the case of transformation as a result of divine revelation, Buber is clear about the nature of this change: the change is permanent, and it involves our very ability to encounter. We are transformed in such a way that we can say "You" to the entire world; we suddenly feel a loving responsibility toward everyone and everything.