"Through the Thou a person becomes I"

This quote by Martin Buber refers to his approach of "I-Thou" relationships. According to Buber, an "I-Thou" relationship is one where two people totally engage with one another, acknowledging each other's full mankind and dealing with each other as equals. In these relationships, a person transcends their private ego and becomes more totally human.

So when Buber says "through the Thou, a person becomes I," he indicates that by engaging fully with another person in an "I-Thou" relationship, a person can transcend their own ego and become a more total variation of themselves. The relationship helps to highlight the best in everyone, permitting them to end up being the best version of themselves possible.