The Assayer & Redondi's "G3"
Michele Camerota's 2008 Complete Dictionary of Scientific Biography entry on Galileo describes this theory in a section entitled "Atomism and the Eucharist":
Atomism and the Eucharist. In section 48 of The Assayer (1623), Galileo set forth a theory of knowledge based on a sharp distinction between “objective” and “subjective” qualities. According to this view, whereas features such as shape, size, position, motion, and number are qualities intrinsic to real things, impressions such as colors, tastes, smells, or tactile properties do not exist in the objects themselves but only in the sentient subject experiencing them. For this reason, sensible qualities were characterized by Galileo as “mere names,” qualities that “reside only in the consciousness” and that would be “wiped away and annihilated” once human sensibility is removed. Behind sensible qualities are the true components of the real world, atoms, whose impinging on the sense organs produces sensory impressions. Hence, for example, the sensation of heat stems from the motion of a “multitude of minute particles” that penetrate human bodies; “their touch as felt by us when they pass through our substance is the sensation we call ‘heat.’” (trans. in Drake, Discoveries and Opinions of Galileo, New York: Anchor Books, 1957, p. 277). Galileo’s stance was clearly rooted in the tradition of ancient atomism, whose most distinguished representatives, such as Democritus and Lucretius, had already stated similar views.
Two documents discovered in the Archives of the Congregation for the Doctrine of the Faith (formerly Holy Office) show that Galileo’s atomistic theory was brought to the attention of the Inquisitorial authorities, most likely before the trial of 1633. The first document was found in 1982 by Pietro Redondi and is usually referred to as “G3,” from the code appearing on the top of its first page. G3 is a denunciation of the atomism of The Assayer. The anonymous author protested that Galileo’s interpretation of sensible qualities clashed with the Catholic doctrine of the Eucharist, according to which, after consecration in the Mass, bread and wine become the body and blood of Jesus Christ. This transformation is understood as transubstantiation because it concerns the substances of bread and wine, whereas their “accidents,” or apparent qualities (color, odor, exterior shape), remain unchanged by virtue of a divine miracle. The author of G3 remarked that, according to the terms of Galileo’s argument, it would be impossible to separate the accidental properties of bread and wine from their own substances. Indeed, because those accidental properties are regarded as “mere names” and as nonexistent outside human sensory perception, on the basis of Galileo’s theory one would be obliged to conclude that “in the Sacrament there are substantial elements of the bread and the wine, which is an error condemned by the Sacred Council of Trent.”
Redondi dated G3 to 1624 and attributed it to Orazio Grassi, the Jesuit mathematician against whom Galileo had written The Assayer. Redondi also connected the document to the trial of 1633, suggesting that the charge of Copernicanism that motivated the trial was a stratagem devised by Pope Urban VIII (a former friend of Galileo) in order to avoid having the scientist face the more serious accusation of Eucharistic heresy.
Redondi’s ascription of G3 to Grassi has been proved to be mistaken, and his thesis concerning the “true” (although disguised) reasons of the trial has been generally rejected by scholars. Nevertheless, Redondi’s book triggered a fresh wave of interest in the Galileo affair and renewed investigations into its cultural and political context.
Another document, similar to G3, was discovered by Mariano Artigas in 1999 and has been carefully studied by Artigas himself along with Rafael Martinez and William Shea. This document is placed in the same volume as G3, the volume EE of the collection “Acta et Documenta,” where it occupies sheet 291. For this reason it has been called “EE 291.”
Like G3, EE 291 is anonymous (it is in Latin while the former is in Italian), and it equally develops a criticism of the theory of sensible qualities expounded in The Assayer, which it deems incompatible with the doctrine of the Eucharist. The author of this document has been identified as the Jesuit Melchior Inchofer, who probably was a member of the commission appointed by the pope in the summer of 1632 to examine Galileo’s Dialogue. Inchofer, a firm opponent of Copernicanism, could have written EE 291 in order to worsen Galileo’s position by adding a further charge against him. Thus, the discoveries of G3 and EE 291, besides providing valuable pieces of information on previously unknown episodes of Galileo’s life, also opened a new chapter of investigation concerning the difficult relationship between atomism and Eucharistic doctrine.
The offensive quote from The Assayer
This (from here) is the offensive quote from They Assayer, in which he denies that the nature of "feather," something beyond the senses, can be known; for him, all knowledge begins and ends in the senses (empiricism):
I move my hand first over a marble statue and then over a living man. To the effect flowing from my hand, this is the same with regard to both objects and my hand; it consists of the primary phenomena of motion and touch, for which we have no further names. But the live body which receives these operations feels different sensations according to the various places touched. When touched upon the soles of the feet, for example, or under the knee or armpit, it feels in addition to the common sensation of touch a sensation on which we have imposed a special name, "tickling." This sensation belongs to us and not to the hand. Anyone would make a serious error if he said that the hand, in addition to the properties of moving and touching, possessed another faculty of "tickling," as if tickling were a phenomenon that resided in the hand that tickled. A piece of paper or a feather drawn lightly over any part of our bodies performs intrinsically the same operations of moving and touching, but by touching the eye, the nose, or the upper lip it excites in us an almost intolerable titillation, even though elsewhere it is scarcely felt. This titillation belongs entirely to us and not to the feather; if the live and sensitive body were removed it would remain no more than a mere word. I believe that no more solid an existence belongs to many qualities which we have come to attribute to physical bodies-tastes, odors, colors, and many more. I believe that no more solid an existence belongs to many qualities which we have come to attribute to physical bodies—tastes, odors, colors, and many more.
Catholic philosophical refutation of the Assayer quote
The Catholic philosopher and historian of science Fr. William A. Wallace, O.P. refutes, in The Modeling of Nature (pgs. 148-149), the above Assayer quote:
Those who hold for the subjectivity of sensible qualities maintain that such qualities have no existence independently of the sensing subject, and on this ground effectively deny the very existence of objective intentions for such qualities. They find convincing Galileo's example of the movement of a feather across the skin to explain the tickle. So they introduce a distinction between primary qualities such as movement and secondary qualities such as the sensed tickle, and hold that the primary qualities have objective existence whereas secondary qualities do not. As a result they populate the universe with particles in motion and attempt to explain all sensations by the various kinds of movement these particles undergo, meanwhile denuding the objective world of sensible qualities in their traditional understanding.
The source of the difficulty here is an improper grasp of the role of the mental representation in the knowledge act. To think of the concept as what is known, rather than seeing that the nature is what is known, though by means of the concept, is to cut oneself off from intellectual knowledge of the real, for one is always left wondering about any extra-mental reality to which the concept might correspond. Similarly, to think of the sensation or the percept as itself what is known, rather than seeing the sensible quality as what is known, though by means of the sensation or percept, is to be imprisoned within one's sense organs and brain. The result is a radical solipsism that prohibits individuals from ever making statements about the objects of experience, leaving them to dwell in a world of their own imaginings.
The tickle may be something sensed on the surface of the skin, but that admission surely does not permit the inference that there is no movement there, or extending the argument further to hold that there is no heat in boiling water, no color in a ruby or a rose, no sound in the cry of a bird, or no odor or taste in an onion. All of these are accidents or accidental modifications of the subjects in which they are sensed. Just as those subjects have natures (inorganic, plan or animal in kind), so accidents may be said to have natures in an analogous sense. And even if we cannot know precisely the nature of heat, of color, and so on, we can at least model those natures in terms of the modalities they introduce in the components of the substantial natures in which they exist, namely the electrons, atoms, and molecules […]
Thus, the issue is not atomism per se but nominalism/idealism vs. realism—or, specially in the Galileo case, it seems: structural realism vs. scientific formalism, where Galileo, Kepler, et al. were structural realists and Bellarmine, Osiander, et al. were scientific formalists, the latter thinking empirical science merely "saves the appearances" rather than, as the former thought, uncovering nature's underlying reality. This is fundamentally the same debate for which Galileo was tried regarding (without sufficient proof at the time) his insistence on the reality of his hypothesis of the earth's motion. It was a philosophical debate, which is why he was condemned for holding a view that was "absurd and false philosophically".
Galileo trial not directly about atomism vs. anti-atomism
St. Thomas Aquinas, the greatest Doctor of the Catholic Church, describing the corruption of a consecrated Host, makes an allusion to atomism in Summa Theologica III q. 77 a. 4 ("Whether the sacramental species can be corrupted?") c. (my emphasis; cf. this answer):
…if the change be so great that the substance of the bread or wine would have been corrupted, then Christ's body and blood do not remain under this sacrament; and this either on the part of the qualities, as when the color, savor, and other qualities of the bread and wine are so altered as to be incompatible with the nature of bread or of wine; or else on the part of the quantity, as, for instance, if the bread be reduced to fine particles [atoms?], or the wine divided into such tiny drops [atoms?] that the species of bread or wine no longer remain.
Fr. Rudjer J. Bošković (1711-1787) was a Jesuit priest and physicist, the "father of modern atomic theory," and he was not condemned for his atomism as Galileo was for his thinking scientific hypotheses can contradict Holy Scripture.
Atomism vs. Dynamism
There was a philosophical debate in Catholic circles regarding atomism vs. dynamism in the 19th and early 20th centuries. Dynamism held that energy is the ultimate reality. Neither doctrine is held by Catholic scholastic philosophers to be a true natural philosophy.
For a good overview of this debate, see:
Cosmology by Édouard Hugon, O.P. (treatise II, second & third articles, pp. 145-162)
Modern Scholastic Philosophy (vol. 1) by Cdl. Desiré Joseph Mercier, 1851-1926, pp. 49-70, 131-144
The Physical System of St. Thomas by Giovanni Maria Cornoldi