3. The generation of the light of the sun is accompanied with the generation of heat. Now every man knows what heat is in himself, by feeling it when he grows hot; but what it is in other things, he knows only by ratiocination. For it is one thing to grow hot, and another thing to heat or make hot. And therefore though we perceive that the fire or the sun heateth, yet we do not perceive that it is itself hot. That other living creatures, whilst they make other things hot, are hot themselves, we infer by reasoning from the like sense in ourselves. But this is not a necessary inference. For though it may truly be said of living creatures, that _they heat, therefore they are themselves hot_; yet it cannot from hence be truly inferred that _fire heateth, therefore it is itself hot_; no more than this, _fire causeth pain, therefore it is itself in pain_. Wherefore, that is only and properly called hot, which when we feel we are necessarily hot. Now when we grow hot, we find that our spirits and blood, and whatsoever is fluid within us, is called out from the internal to the external parts of our bodies, more or less, according to the degree of the heat; and that our skin swelleth. He, therefore, that can give a possible cause of this evocation and swelling, and such as agrees with the rest of the phenomena of heat, may be thought to have given the cause of the heat of the sun. It hath been shown, in the 5th article of chapter XXI, that the fluid medium, which we call the air, is so moved by the simple circular motion of the sun, as that all its parts, even the least, do perpetually change places with one another; which change of places is that which there I called fermentation. From this fermentation of the air, I have, in the 8th article of the last chapter, demonstrated that the water may be drawn up into the clouds.