The subject of natural science moving matter, bodies. Bodies cannot be separated from motion, their forms and kinds can only be known through motion, of bodies apart from motion, apart from any relation to other bodies, nothing can be asserted. Only in motion does a body reveal what it is. Natural science therefore knows bodies by considering them in their relation to one another, in motion. The knowledge of the different forms of motion is the knowledge of bodies. The investigation of these different forms of motion is therefore the chief subject of natural science. You being seated there at the centre of the natural sciences will be in the best position to judge if there is anything in it.
Letters: Marx-Engels Correspondence 1873