Concepts as powers
The concept of 'I', that of the cogito, works as a concept through its application to a range of components, through the inseperability of those components from the concept. Thinking is concieved as belonging to an I or otherwise, as a posession of an I. Similarly, being, which only achieves certitude through being thought, must therefore be the property of an I that thinks. In a Cartesian world, 'thinking' without the I loses its sense, is hardly an activity at all, is a free-floating component, with no definite concept. Clearly it is still an 'activity', or at the very least some kind of event. But they are vague and empty concepts, place-holders that absorb differences.
In this way we can see that being a concept means having a power over a set of components, of raising issue with them. The power of 'I' is to ask the question of possesion, in fact and by right, of everything. Leading directly to the transcendental I as the concept is developed.