Galileo on Corpuscularianism (1623)

In the early 1600’s, a certain view of matter swept across the academic world, to be adopted by nearly every major scientific or philosophical figure for at least the next 100 years. That view was corpuscularianism, a resurgance of ‘atomism’ held by ancient Greek philosophers such as Epicurus and Lucretius, the view that the physical world is composed of tiny particles, atoms, or corpuscles, and that nearly everything one observes can be explained in terms of these. Galileo (of physics and astronomy fame) explains and defends this view in The Assayer.

1. On Tickling: Up until Galileo’s time, it was thought that all observed properties of material objects actually resided in those objects. Not only does an apple have solidity and roundness, but also redness, sweetness, and so on. Lemons have sourness in them. Fires have heat in them. And so on. Galileo challenged this claim:

“I strongly suspect that the commonly held conception of the matter is very far from the truth, inasmuch as heat is generally believed to be a true accident, affection, or quality which actually resides in the material which we feel to be heated.”

[Historical note: Why did they think this? Throughout the medieval period, it was assumed that an object could only IMPART a property if it already HAD that property. For instance, a fire imparts heat to a kettle only because the fire itself HAS heat. Similarly, a tomato only imparts redness into your mind because it too HAS redness. It was thought that the “form” of redness emanated from the object and “stamped” itself onto your mind like a seal on hot wax. Other scientific explanations were even spookier. For instance, a sedative was said to work in virtue of its “soporific power” – the power of putting one to sleep.]

Galileo considers an experiment: Imagine running your hand over someone who is ticklish. They experience the titillating sensation of tickling. But, nowhere in your hand is there anything like tickling! All that the hand contributes is movement and contact:

“And it seems to me that he would be gravely in error who would assert that the hand, in addition to movement and contact, intrinsically possesses another and different faculty which we might call the ‘tickling faculty,’ as though tickling were a resident property of the hand per se.”

Similarly, if one is tickled by a feather, we do not think that somehow the tickling is IN THE FEATHER, or that the feather somehow possesses a real power called “the tickling faculty”. Rather, all the feather seems to contribute are certain sizes, shapes, textures, and motions of its parts, while the SENSATION of being tickled is merely within US.

2. The “Real Properties” of Bodies: From here, Galileo speculates that things like color, taste, odor, etc. are ALL like this; that is, like tickling, do not exist in the objects themselves – but rather “have really no existence save in us”. He writes,

“Now this titillation is completely ours and not the feather’s, so that if the living, sensing body were removed, nothing would remain of the titillation but an empty name. And I believe that many other qualities, such as taste, odor, color, and so on, often predicated of natural bodies, have a similar and no greater existence than this.”

It is at least CONCEIVABLE that objects do not really have these properties. For, while we cannot coherently imagine a material thing without size, shape, or degree of motion, we CAN imagine one without color, odor, or sound. (Give it a try: Imagine an apple that is silent, not sweet, gives off no odor, and has no color. It’s possible, right?) Thus, material objects are at least LOGICALLY separable from these properties. Galileo writes,

“Now, whenever I conceive of any material or corporeal substance, I am necessarily constrained to conceive of that substance as bounded and as possessing this or that shape, as large or small in relationship to some other body, as in this or that place during this or that time, as in motion or at rest, as in contact or not in contact with some other body, as being one, many, or few—and by no stretch of imagination can I conceive of any corporeal body apart from these conditions. But I do not at all feel myself compelled to conceive of bodies as necessarily conjoined with such further conditions as being red or white, bitter or sweet, having sound or being mute, or possessing a pleasant or unpleasant fragrance. On the contrary, were they not escorted by our physical senses, perhaps neither reason nor understanding would ever, by themselves, arrive at such notions. I think, therefore, that these tastes, odors, colors, etc., so far as their objective existence is concerned, are nothing but mere names for something which resides exclusively in our sensitive body, so that if the perceiving creatures were removed, all of these qualities would be annihilated and abolished from existence.”

It seems that objects MUST have the following:

 Size  Shape  Location in space and time  Motion or rest  Number (or quantity)

Galileo calls these the “primary and real properties”. Things like color, taste, and odor, he calls the “sensible qualities.”(Note: Later, we will discuss this distinction in detail, as made by John Locke. He calls the former “primary qualities” and the latter “secondary qualities.”) 2

3. Corpuscularianism: From this unmysterious explanation of the tickling sensation, it is a short journey to similar explanations for all other sensations. Galileo concluded that, just as the shape of the parts of the feather, the speed with which it is drawn across the skin, etc., seemed to be what produced the tickling sensation, so too the shapes, sizes, speeds, and so forth, of the particles being emitted by material objects must be what produce the different sensations of colors, sounds, flavors, etc. in us:

“Thus our tastes are caused, pleasant or harsh in accordance with variations in the contact of diversely shaped particles, and depending upon whether they are few or many, and whether they have high or low velocity.”

Heat: For instance, he speculates that HEAT is also like this. He thought that perhaps the difference between a pleasant warmth and a painful burning may be the difference between the particles of a fire or heat source moving more slowly or more quickly. (Note: Francis Bacon actually proposed this 3 years earlier, in 1620.)

Light: He even speculates that light itself is merely the smallest particle—arrived at by breaking up matter into smaller and smaller bits until you reach the very smallest, indivisible thing:

“once we arrive at the point of ultimate and maximum dissolution into truly indivisible atoms, light itself may be created, with an instantaneous motion … capable, I say, of filling vast spaces.”

Conclusion: This conclusion may seem trivial and obvious, but this was a revolutionary way of thinking in the 16th and 17th centuries, and it marked a huge leap forward for science and philosophy alike. Yet, as we shall see, it inadvertantly led to several skeptical worries. Galileo foreshadows the coming age:

“But I should not like, … while removing one doubt, to give birth to a hundred more, as I fear might in part be the case even in this timid venture from shore.”