It was a request I couldn't refuse. Alan Yentob, the documentary maker, was doing a programme on do-it-yourself psychotherapy: self-help. In the course of his research he had come across the work of the late Viktor Frankl, and saw on the back of one of his books that I had written an endorsement. Would I be willing to talk about the man and his ideas?

When it comes to psychotherapy I am strictly an amateur, but I agreed, because for me Frankl had discovered a set of truths about the human condition that no one had stated so clearly, and the circumstances in which he did so were utterly extraordinary. His work is still not well enough known. So I agreed.

Frankl survived three years in Auschwitz, Dachau and other concentration camps. That in itself was something of a miracle. But he did more than survive. He helped others survive. On the basis of his experiences during the Holocaust he founded a new school of psychotherapy — he called it Logotherapy — whose central idea is summed up in the title of his most famous book, Man's Search for Meaning.

Frankl's first discovery was that in Auschwitz you needed a reason to carry on living. If you lost that, you lost the will to live, and if you lost that, you died. The conditions in the camp were designed to break not just the body, but also the spirit. They were so dehumanising that they turned the prisoners into the walking dead. It was the second discovery that changed Frankl's life with the force of revelation. The Nazis robbed the prisoners of every vestige of humanity: their possessions, their clothes, their hair, even their names. Instead they were given numbers, tattooed on their arms. But Frankl realised that there remained one thing they could not take away: the freedom to choose how to respond. That one slender opening of hope in the walls of despair was all that remained, and it was enough.

Frankl saw that you had to redefine your situation if you were to stay sane. The way he did so was to persuade himself that he was not a prisoner in a concentration camp but a psychotherapist taking part in an experiment. This allowed him to salvage a vestige of freedom and dignity. He had found a purpose in life. He now knew that his task was to do all he could to rescue his fellow prisoners from despair. He did so by helping them to find a reason to live, a task not yet completed, work still to be done. For one it was completing a series of travel guides, for another it was rejoining a child in Canada who needed him. That sense of a mission not yet fulfilled gave people the strength to carry on. Frankl was fond of quoting Nietzsche's remark that those who have a Why can withstand almost any How.

He tells the story of one young woman in the camp who was about to die. Unbelievably, she was cheerful. She explained: until then she had never thought about matters of the spirit. But in Auschwitz she had nothing except a tiny window through which she could see a branch of a chestnut tree on which there were two blossoms. The branch seemed to speak to her: “I am here — I am life, eternal life.” She was there to experience that.

Frankl discovered that to find meaning in life we must ask not what do I need from life, but what does life need from me? Each of us is here for a purpose, a task, that only we can fulfil. To find meaning is to find that task.

There is something deeply spiritual about Frankl's work. The self finds itself by attending to something beyond the self. Our reason to be comes in the form of a call, a summons, a vocation, a mission, the voice of the beyond-within. Religious people call this the voice of God, but you do not have to be religious to hear it.

Frankl's writings are an antidote to today's materialists who would reduce the human condition to biological imperatives and the human person to no more than a handful of dust. It was his gift to be able to find, and in some cases to help create, epiphanies in the heart of darkness, fragments of Heaven even at the gates of Hell.

Rabbi Sir Jonathan Sacks is the Chief Rabbi of the United Hebrew Congregations of the Commonwealth