Descartes turns to fire and he knows only two sources: the stars in the heaven and the terrestrial fire on earth below. Since the stars are out of reach, he suggests, we look at the fire below, say, at a piece of burning wood. If you are expecting him to carry out a rigorously experimental and quantitative analysis of the modes of combustion, he is drawing us to what lies below the threshold of vision inferring Nature. When the wood burns, he says;
we see at a glance, that it moves small particles of this wood... Someone else, if he pleases, may imagine in this wood the form of fire, the quality of heat, and the action that burns it as different things. As for me, who am afraid of deceiving myself if I suppose anything more to be there than what I see must necessarily be present, I am content with conceiving the movement of its parts.
In other words, motion is not only a necessary, but a necessary and sufficient condition for fire.
But if motion is all that is required to make a body fluid and to cause the sensation of fire, why aren't we scorched by the breeze ? To this, Descartes replies:
We must consider not only the speed but the size of the moving parts. The smaller ones produce the more fluid bodies, but the larger ones have more power to burn and, generally speaking, to act upon other bodies.
Until Lavoiser at the end of 18th century said - fire was a genuine physical substance like water or air. Where we see a process of oxidation, namely the combination of oxygen with another substance, and the concomitant release of light and heat, they saw a manifestation of properties of fire.
Want to learn more?.... Read 'Science in the West and India' - Some Historical Aspects' by B V Subbarayappa (Indian Institute of World Culture, Bangalore) and N Mukunda (IISc, Bangalore)
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