In 1940 he returned to Europe to join the British Army which had been arranging a special brigade for German Jews wanting to fight against Hitler.

Immediately after the war he returned to Mönchengladbach to search for his mother but found that she had been sent to the gas chambers in the Auschwitz concentration camp.

He formulated a new and distinctive supreme moral imperative: "Act so that the effects of your action are compatible with the permanence of genuine human life".

[9] While The Imperative of Responsibility has been credited with catalyzing the environmental movement in Germany, his work The Phenomenon of Life (1966) forms the philosophical undergirding of one major school of bioethics in America.

Heavily influenced by Martin Heidegger but also one of Heidegger's most outspoken philosophical critics,[10] The Phenomenon of Life attempts to synthesize the philosophy of matter with the philosophy of mind, producing a rich existential understanding of biology, which ultimately argues for a simultaneously material and moral human nature.

She carries a human trust, and we should not make abortion merely a matter of her own private wish", society had a "social responsibility" to pregnant mothers, and "To give this mission[motherhood] over completely to individual choice oversteps the order of nature.

"[12] His writing on the history of Gnosticism revisits terrain covered by earlier standard works on the subject such as Ernesto Buonaiuti's Lo gnosticismo: storia di antiche lotte religiose (1907), interpreting the religion from a unique version of existentialist philosophical viewpoint that also informed his later contributions.