In Being and Nothingness, Sartre discusses negation in a number of contexts, but I cannot for the life of me figure out a definition of it--even a rough one. So far as I can tell, it is not merely a logical operation and it is not just a psychological attitude. Does anyone have any insights or suggestions for me to understand this better?
If I understand this correctly, Sartre considers negation (usually as internal negation) as one of the fundamental aspect of humanity, because it reflects the tension between being-in-itself and being-for-itself where the latter is (ostensibly) a feature specific to human consciousness. 'Being-in-itself', for Sartre, is the totality of what someone is: the accumulated sum of one's biology, history, and thoughts and behavior as they manifest in the present moment. 'Being-for-itself', by contrast, is the aggregate of how we perceive ourselves: what we think of ourselves, what we want for ourselves, what we imagine ourselves to be... As you can see, the second of these often amounts to a rejection or denial of the first: we want to be the kind of person who does X, so we reject the fact that (historically) we may have done Y.
That is the sense of 'negation' in Sartre: negating that which is historically (observably) the case in support of an aspirational or ideal view of the self. It's that philosophical tension that drives learning, growth, ethics, and etc.