As we know of mass through a definite sense-perception, so we know of heat. In the latter case we rely on the sense of warmth. In Chapter VIII we took the opportunity to test the objectivity of the information received through this sense. Still, one-eyed, colour-blind observation is naturally unable to take account of these sense-messages. To this kind of observation nothing is accessible, we know, except spatial displacements of single point-like entities. Hence we find Bacon and Hooke already attributing the sensation of warmth to minute fast-moving particles of matter impinging on the skin. Some time later we find Locke taking up the same picture. We see from this how little the mechanical theory of heat owes to empirical facts. For even in Locke's time the connexion between heat and mechanical action, as recognized to-day, was completely unknown.