In Being and Nothingness, considered as the philosopher's greatest work, Jean-Paul Sartre writes on the issue of the pen, and the hand:
I do not apprehend my hand in the act of writing but only the pen which is writing; this means that I use my pen in order to form letters but not my hand in order to hold the pen. I am not in relation to my hand in the same utilizing attitude as I am in relation to the pen; I am my hand. That is, my hand is the arresting of references and their ultimate end. The hand is only the utilization of the pen.
In the act of writing it is the point of the pen which I look at in synthetic combination with the line or the square marked on the sheet of paper. But my hand has vanished; it is lost in the complext system of instrumentality in order that this system may exist. It is simply the meaning and the orientation of the system.
In this sense the structure of the world implies that we can insert ourselves into the field of instrumentality only by being ourselves an instrument, that we can not act without being acted on.
We do not use this instrument, for we are it. It is given to us in no other way than by the instrumental order of the world... I do not have to adapt myself to it nor to adapt another tool to it, but it is my very adaptation to tools, the adaptation which I am.
Jean-Paul Sartre, Being and Nothingness, first published 1943.