The Nature of Heat and Light

by Rene Descartes

Light naturally exists in the stars and flame, or fire.

Movement in Parts

When flame burns wood, we see that it moves the small parts of the wood and separates them from one another.

• This transforms the subtler parts into fire, air, and smoke, and leaving the grosser parts as ashes.

Body A cannot move Body B unless Body A itself were also moving [7].

It means that the flame’s body that acts against the wood is composed of small parts*. These move independently of one another with a very fast and violent motion.

*Superphysics note: Unlike Newton who viewed motion from a large viewpoint, as from the perspective of ‘Lord God’, Descartes views movement from the quanta or small parts

They push and move with them the parts of the body that they touch and that do not offer much resistance.

Its parts move independently of one another. Even though they often act together to create a single effect, they nonetheless act on their own against the bodies they touch.

Their motion is very fast and very violent because they are so small that we cannot distinguish them by sight. They would not have the force to act against other bodies if the quickness of their motion did not compensate for their smallness.[8]

I add nothing concerning the direction in which each moves.

This is because the power to move and the power that determines that motion’s direction are two completely different things. These can exist one without the other (as I have set out in the Dioptrics)[9]. Each part moves in the way least difficult* for it by the bodies around it.[10]

*Superphysics note: This is the path of least resistance for each quantum

A flame can have parts going upward, downward, in straight lines, or circles. They can go in all directions, without changing anything of the flame’s nature.

Thus, if you see almost all of them tending upward, then it means that the bodies around them offer the least resistance in the upward motion.

How does the flame heat us and give light for us?

Our sensation of heat becomes pain when flame is violent, and tickling when flame is moderate.[11]

This tickling and pain only comes from our mind. Thus, everything we touch also comes from our mind.

Many experiences favor this opinion. For example, by rubbing our hands together we heat them without any fire. The same motion that is in the flame causes us to sense light.