Full Idea
There is the being which in the phenomenality of existence is not but can be, ought to be, and therefore decides in time whether it is in eternity. This being is myself as 'Existenz'.
Gist of Idea
'Existenz' is the potential being, which I could have, and ought to have
Source
Karl Jaspers (Philosophy [1932], vol.2)
Book Reference
'Existentialism', ed/tr. Solomon,Robert C. [Modern Library 1974], p.135
A Reaction
This is quintessentially existentialist, in its claim that my mode of being could be quite other than it is. Personally I aim to fulfil the being I've got. Play the cards you have been dealt.