In the preceding chapter we gained a new insight into the relationship between mass and force. We have come to see that our concept of force is grounded on empirical observation in no less a degree than is usually assumed for our concept of number, or size, or position, provided we do not confine ourselves to non-stereoscopic, colourless vision for the forming of our scientific world-picture, but allow other senses to contribute to it. As to the concept mass, our discussion of the formula F=ma showed that force and mass, as they occur in it, are of identical nature, both having the quality of force. The factors F and m signify force in a different relationship to space (represented by the factor a). This latter fact now requires some further elucidation.