Full Idea
The Dasein which I myself am in each instance is defined in its Being by my being able to say of it, 'I am, that is, I can'.
Clarification
'Dasein' roughly means 'existence' or 'being there'
Gist of Idea
My active existence is defined by being able to say 'I can'
Source
Martin Heidegger (History of the Concept of Time [1925], p.298), quoted by Richard Polt - Heidegger: an introduction 3.§31-3
Book Reference
Polt,Richard: 'Heidegger: an introduction' [Routledge 2003], p.69
A Reaction
I like this emphasis on the more active aspect of the Cogito idea. The whole Enlightenment account of things has them as inert, and falling under laws and theories. The Aristotelian account makes potentiality central.