When we suppose that heaviness is a real quality of which all we know is that it has the force to move the body that possesses it towards the center of the earth, we find no difficulty in conceiving how it moves the body or how it is united to it. We do not suppose that the production of this motion takes place by a real contact between two surfaces, because we experience in ourselves that we have a specific notion to conceive it by. I think that we misuse this notion when we apply it to heaviness, which as I hope to show in my physics [ie the yet to be published Principia Philosophiae], is not anything really distinctly from body; but it was given to us for the purpose of conceiving the manner in which the mind moves the body.
Descartes to Elisabeth May 21, 1643
Editor in Chief,
Damon P Meriweather