From John Paul Sartre Existentialism as a Humanism,

As an example by which you may the better understand this state of abandonment, I will refer to the case of a pupil of mine, who sought me out in the following circumstances. His father was quarrelling with his mother and was also inclined to be a "collaborator"; his elder brother had been killed in the German offensive of 1940 and this young man, with a sentiment somewhat primitive but generous, burned to avenge him. His mother was living alone with him, deeply afflicted by the semi-treason of his father and by the death of her eldest son, and her one consolation was in this young man. But he, at this moment, had the choice between going to England to join the Free French Forces or of staying near his mother and helping her to live. He fully realised that this woman lived only for him and that his disappearance -- or perhaps his death -- would plunge her into despair. He also realised that, concretely and in fact, every action he performed on his mother’s behalf would be sure of effect in the sense of aiding her to live, whereas anything he did in order to go and fight would be an ambiguous action which night vanish like water into sand and serve no purpose. For instance, to set out for England he would have to wait indefinitely in a Spanish camp on the way through Spain; or, on arriving in England or in Algiers he might be put into an office to fill up forms. Consequently, he found himself confronted by two very different modes of action; the one concrete, immediate, but directed towards only one individual; and the other an action addressed to an end infinitely greater, a national collectivity, but for that very reason ambiguous -- and it might be frustrated on the way. At the same time, he was hesitating between two kinds of morality; on the one side the morality of sympathy, of personal devotion and, on the other side, a morality of wider scope but of more debatable validity. He had to choose between those two. What could help him to choose? Could the Christian doctrine? No. Christian doctrine says: Act with charity, love your neighbour, deny yourself for others, choose the way which is hardest, and so forth. But which is the harder road? To whom does one owe the more brotherly love, the patriot or the mother? Which is the more useful aim, the general one of fighting in and for the whole community, or the precise aim of helping one particular person to live? Who can give an answer to that a priori? No one. Nor is it given in any ethical scripture. The Kantian ethic says, Never regard another as a means, but always as an end. Very well; if I remain with my mother, I shall be regarding her as the end and not as a means: but by the same token I am in danger of treating as means those who are fighting on my behalf; and the converse is also true, that if I go to the aid of the combatants I shall be treating them as the end at the risk of treating my mother as a means. If values are uncertain, if they are still too abstract to determine the particular, concrete case under consideration, nothing remains but to trust in our instincts. That is what this young man tried to do; and when I saw him he said, "In the end, it is feeling that counts; the direction in which it is really pushing me is the one I ought to choose. If I feel that I love my mother enough to sacrifice everything else for her -- my will to be avenged, all my longings for action and adventure then I stay with her. If, on the contrary, I feel that my love for her is not enough, I go." But how does one estimate the strength of a feeling? The value of his feeling for his mother was determined precisely by the fact that he was standing by her. I may say that I love a certain friend enough to sacrifice such or such a sum of money for him, but I cannot prove that unless I have done it. I may say, "I love my mother enough to remain with her," if actually I have remained with her. I can only estimate the strength of this affection if I have performed an action by which it is defined and ratified. But if I then appeal to this affection to justify my action, I find myself drawn into a vicious circle.

Moreover, as Gide has very well said, a sentiment which is play-acting and one which is vital are two things that are hardly distinguishable one from another. To decide that I love my mother by staying beside her, and to play a comedy the upshot of which is that I do so -- these are nearly the same thing. In other words, feeling is formed by the deeds that one does; therefore I cannot consult it as a guide to action. And that is to say that I can neither seek within myself for an authentic impulse to action, nor can I expect, from some ethic, formulae that will enable me to act. You may say that the youth did, at least, go to a professor to ask for advice. But if you seek counsel -- from a priest, for example you have selected that priest; and at bottom you already knew, more or less, what he would advise. In other words, to choose an adviser is nevertheless to commit oneself by that choice. If you are a Christian, you will say, Consult a priest; but there are collaborationists, priests who are resisters and priests who wait for the tide to turn: which will you choose? Had this young man chosen a priest of the resistance, or one of the collaboration, he would have decided beforehand the kind of advice he was to receive. Similarly, in coming to me, he knew what advice I should give him, and I had but one reply to make. You are free, therefore choose that is to say, invent. No rule of general morality can show you what you ought to do: no signs are vouchsafed in this world. The Catholics will reply, "Oh, but they are!" Very well; still, it is I myself, in every case, who have to interpret the signs. . .