Is heat a quality of the matter which we feel when it is heated or it is in a sensation of a body? In The Assayer, Galileo starts with the proposition “Motion is the cause of heat”. So, he wants to undermine a commonly held conception that heat is a quality which resides in the material.

Galileo argues that tastes, odors, colors etc. do not truly exist comparing to primary and real properties of the matter. If perceiving creatures were removed, all these sensitive qualities would be abolished from existence. So, they are nothing but mere names for something which resides exclusively in our sensitive body. For instance, I pass my hand over a marble statue and over a living man. In both cases, the primary qualities of motion and contact of my hand will similarly affect the two objects. If I touch a living body, it will be affected in various ways, depending upon the part of the body I happen to touch. Let’s say I touch somebody with a feather under armpit. This would give to a living body we touch a feeling which we call tickling. We could assert that my hand intrinsically possesses the tickling faculty, in addition to movement and contact. However, should I touch other places, there would not be anything similar to titillation. That is why this latter affection is completely ours and not the feather’s or the hand’s.

How then can we explain that some tastes are pleasant, others are harsh? Galileo states that certain material bodies are formed into tiny particles. Received by the upper surface of the tongue, they blend with its substance and moisture. Depending on the quantity, velocity and variations in the contact of diversely shaped particles, we may receive a pleasant or harsh taste. Similarly, smell is caused by particles that enter the nostrils and penetrate the various nodes. Through motion and contact with the instruments of smell, these particles produce savouriness or unsavouriness. As for the sound, it can be explained by a rapid vibration of air. Our ears receive waves which vibrate certain cartilages of a certain drum. So, high frequencies produce high tones, while low frequencies produce low ones.

Returning to the heat, Galileo believes that those fire materials which produce and make felt in us the sense of heat consist of a multitude of tiny particles of different shape and velocity. So, it is by their contact with our body, we feel what we call “heat”. Depending upon the number and the velocity of penetrating particles, the heat will be pleasantly warm or unpleasantly hot. The fire is thus operated by the movement of its particles. Depending upon the greater or lesser density of the bodies concerned, the particles of flame dissolve them. Many of them are transformed into further corpuscles of flame. Thus, some further quality which we call “heat” does not exist in fire. This affection is caused in us by movement of tiny particles of flame which penetrate our bodies. Once this movement ceases, their operation upon us will stop.  Since the movement is required and the mere presence of corpuscles of flame is not by itself sufficient in order for heat to be stimulated in us, Galileo declares motion to be the cause of heat.

Galileo concludes that apart from shape, number, movement, penetration, and contact, some further qualities such as taste, odor, color, heat etc. are nothing but simple words. These are secondary qualities of the matter that do not exist in external bodies. If ears, tongues, eyes, noses be taken away, the primary qualities such as shape, number, movement of bodies would remain since they exist by themselves. This is not the case for secondary qualities such as tastes, sounds, odors, colors , heat which depend on sensitive bodies.

Bibliography:

  • Galileo, The Assayer in The Scientific Background to Modern Philosophy, Selected Readings, Edited by Michael R. Matthews, Hackett Publishing Company, Indianapolis/Cambidge, 1989.