While a prisoner of war in 1940 and 1941, Sartre read Martin Heidegger's "Being and Time" (1927), which uses the method of Husserlian phenomenology as a lens for examining ontology. Sartre attributed the course of his own philosophical inquiries to his exposure to this work. Though influenced by Heidegger, Sartre was profoundly skeptical of any measure by which humanity could achieve a kind of personal state of fulfillment comparable to the hypothetical Heideggerian "re-encounter with Being". In Sartre's account, man is a creature haunted by a vision of "completion" (what Sartre calls the "ens causa sui", meaning literally "a being that causes itself"), which many religions and philosophers identify as God. Born into the material reality of one's body, in a material universe, one finds oneself inserted into being. In accordance with Husserl's notion that consciousness can only exist as consciousness "of" something, Sartre develops the idea that there can be no form of self that is "hidden" inside consciousness. On these grounds, Sartre goes on to offer a philosophical critique of Sigmund Freud's theories, based on the claim that consciousness is essentially self-conscious.