As a phenomenologist, Sartre does not aim at arguing ( argument may prove anything) , but at describing the phenomena, what shows itself in a compelling fashion. In ordre to assess Sartre's philosophy, you cannot limit yourself to Existentialsme est un humanisme, but need to read L'Etre et le néant ( with a good guidesuch as Gardner's).
Short answer to this :
"What is the formal definition of existence, God, (human) essence".
Existence is precisely the mode of being of a being that has no definition, no essence. Hence, existence is " the being of nothingness" .
- 3 historical stages in the constitution of the concept of existence :
(1) Existence as actuality , or being actually, not simple being potentially (traditional aristotelian ontology)
(2) Existence as actuality of a finite being ( etymologically) standing out (" ex") of its causes ( Monothesitic theology for which God *is a being * but does not " exist")
(3) Existence as actuality of a finite being that is not a "thing" , a natural being ( having an essence), a subject or substance bearing properties --> existentialist concept of existence
or, in Kierkegaard's words, actuality of a being that is neither eternal ( infinite) , neither in time understood as mere succession of identical "
nows" ( the time of existence being the " instant")
or, in Heidegger's words , existence is the mode of being peculiar to "Dasein" for whom being is not " being this" ( having an " essence"
, a " quiddity") but " having to be ( oneself)" in such a way that "
its essence lies in its existence" ( contrary to God whose existence
lies in its essence).
Note : to exist, in the existentialist sense, is not to be a " man" ; there is a conceptual difference between " Dasein" and " man" for a man is a natural being, a living being belonging to a species, etc, with a nature, etc.
- 3 features of existence in the third sense
a) ipseity : an "existant" ( a being that " exists") is a self ; as says Heidegger , Dasein is this being that is always and everytime mine ; to exist is to be a " who" not a " what" ( an instance of a universal model);
b) facticity ( being thrown in the world, having being as a given, as a fact , in a given situation)
c) transcendance or " project" : being able to go beyond one's situation , to define oneself freely through one's own life plan and actions , to direct oneself towards being-possibilities
Link : https://plato.stanford.edu/entries/existentialism/#FacTra
- According to Sartre, existence is a mode of being that violates the principle of non-contradiction : the " self" or "for-oneself" ( " pour-soi")
" is what it isn't, and is not what it is" ,
for the self cannot be anything whithout being aware of being such; but as a " self watcher" can never be perfectly identical to the self whereof he is aware; consciousness implying the invisible distance of " negation " or " nothingness".