Whether Sartre manages to offer a viable ethics is a complicated question that needs to be broken down. To ask whether Sartre offers ‘viable ethics’, is to ask whether the set of ethical principles he proposes as a whole are viable.

To answer the question of whether Sartre’s ethics are viable, we must develop some criteria about what defines an ethical system (a set of ethical principles) as ‘viable’:

First, let us distinguish the difference between viability and practically. For an ethical principle to be viable, it must be practical. That is, if a set of ethical principles cannot be applied to real life scenarios (practicality), it isn’t viable.

Next, the viability of an ethical system is also characterized by its consistency; there must be no logical contradictions, every ethical principle should be justified and supported by each other. The application of said principles should also be applicable in a predictable and consistent manner, the ‘right’ thing to do in one scenario should still be the right thing to do in a similar situation unless there’s an adequate reason for an exception. Without this type of consistency, ethics will not be viable as it will not be a reliable way to solve ethical conflicts and disagreements; ethical principles that contradict themselves will not be able to provide clear solutions to specific ethical conflicts.

These two criteria are what I believe to be, the most human, and important factors in determining whether an ethical system is viable. If an ethical system is to not be practical and consistent, then I do not believe a man can viably believe and live by it. Man cannot abide by a system in which he cannot apply. Man cannot abide by a system in which he cannot trust. Just as you cannot trust a friend who is inconsistent in their loyalty to you, you cannot trust an ethical system that is inconsistent in its resolutions to conflicts. If you don’t trust a system, you cannot (in good faith) abide by it.