Do you think there must be a human nature? If such a thing exists, at least a relatively fixed one, then true scientific understanding is possible. Because people, with a very limited amount or set of experiences, are capable of learning their own language, as well as use it creatively, then there must be some sort of “bio-physical structure” within the mind that enables individuals to deduce a unified language from the multiplicity of individual experiences.
If you agree with this conclusion then you are a typical Cartesian rationalist. You will answer questions with appeals to universal human nature and reason, the standard for intellectuals to judge and rationalize a precise conceptualization for a better society and the future. Violence, injustice, falsehood, etc., must be fought for the sake of justice, because there must be a guiding principle, a fixed and rational standard for knowledge, society… Otherwise we cannot judge which actions or claims are true or correct or just or determine which actions to take.
On the other hand, if you think human nature does not exist, because “existence precedes essence,” then there are no rational principles or a priori values. No transcendent values exist prior to choice, ready-made and perfect, waiting for your consent in your choice how to live, for you are nothing else but what you choose to make of yourself. Consequently you are responsible of yourself and for everyone else. You create a certain image of a person of your choices, through your actions. In choosing yourself you choose humanity. What you do contributes to humanity in general, hence, you are responsible of all mankind. Once you become aware that your choices actually chooses for everyone else, you will experience a deep-seated anxiety. Once you realize that there is no God, nobody with infinitely perfect wisdom who knows the “Right choice,” then your choice is completely your responsibility, your creation. Your choice cannot depend on anyone else. If there is no pre-existing human configuration, no perfect society, no standard of precise justice, then you are condemned to always choose freely.
However, if you wished to avoid starting with the abstract question by starting in a different direction and pose this – how does the concept of human nature function within society – then you evade the requirement of making claims of universal truths, and instead choose to historicize grand abstractions. This method distinguishes the actual operational categories within a discipline from a broad conceptual marker, like “life,” in which themselves may have had of little import regarding the internal evolution of the scientific disciplines. Therefore, because there is no external position or universal understanding that is beyond history and society, the task is to historicize the universal categories each time they’re encountered. I.e., it was not through the study of human nature that discoveries were possible, such as linguists with “consonant mutation” or psychologists, the “principle of analysis of dreams,” or anthropologists, the “structure of myths.” This way, the “why” questions are transformed into “how.” In the attempt to answer “why,” you are restricted to models of justice, knowledge, truth, or search for general principles that evaluate the condition or situation. Perhaps you realize this attempt actually hides the concrete function of power. The notion of truth or justice or morality has been concocted in order to function as an instrument of a political and economic power. For knowledge is already soaked within conflicts small and large, not external or above the disagreements, or discord.