2. Sensitive Soul/Body

Let us notice that, no matter how many Aristotelian ideas he modified, Galileo’s conception of perception remained partly Aristotelian. According to Aristotle’s hylomorphism, the unification of the soul with primary matter can result in three kinds of living beings. Plants possess only a nutritive soul; thus, they are merely alive. Animals have a sensitive soul as well thus they are alive and capable of perception and motion. Finally, humans, in addition to the first two, also possess an intellectual soul (mens, mind) enabling them to be alive, perceive and think (Aristotle DA 415a200–25; 434a22‒b4; 429a9‒10, 434b3; Wilkes, Reference Wilkes, Nussbaum and Rorty1995). Reading Galileo, we see that while geometrical properties are apprehensible to the intellect, perception is taken care of by a more rudimentary psychological capacity: the ‘animate and sensitive body’ [corpo animato e sensitivo] (Galileo, Reference Galilei, Drake and O’Malley1623, p. 310; cf. Buyse, Reference Buyse2015). Perceptions are acquired by the sensitive body and are transferred only subsequently to the memory, imagination, and intellect (DA 403a5–403b18). That is, perception itself, particularly the comprehension of non-geometrical properties, is not executed by the mind (mens), it presupposes the cognitive involvement of the living sensitive body.

It is essential to bear in mind this Aristotelian inheritance of Galileo because his post-Cartesian readers have frequently taken for granted that the discrimination of properties implied a Cartesian-type mind. I argue that this (mis)understanding has rendered Galileo’s argument appear flawed. The sensitive soul/body to which he refers is a simpler, if you like more primitive cognitive capacity than the mind. Although, at first glance, Galileo’s partial retention of Aristotelian cognitive psychology may seem to preclude it, I will demonstrate that it is precisely the unsophistication of the sensitive soul that provides the chance to understand his distinction between geometrical and sensible qualities.

2.1 Matter and Form

The Aristotelian conception of perception comprised two fundamental principles.

This conception was intuitively understandable. Aristotle’s spatial analogy was lucid: the matter of the sense organ takes on the form of the perceived thing without its matter, just as the wax receives the impression of a signet ring. The perception of proper sensibles, such as colour or smell, was less straightforward but the framework facilitated an analogical understanding: the substantial form transmits the entire nature of the perceived object.

The shift from hylomorphism to corpuscularism in early modern philosophy disrupted the established correspondence between metaphysics and the psychology of perception. With the abandonment of substantial form, formal causes were replaced by simple mechanical ones, which were not sufficiently complex (and vague) to explain the perception of non-spatial properties. Corpuscles, which lack non-spatial properties, exert mechanical effects that reconfigure the corpuscular structure of the sense organ. The resulting physical structure could represent the spatial shape of the impinging corpuscles. This can be taken as a mechanical version of the Aristotelian conception. However, simple mechanical effects fail to account for how perceptions of non-spatial, even non-existent sensible qualities arise. Consequently, the new metaphysical framework entailed two anti-Aristotelian consequences:

• • According to the corpuscularists, things did not possess non-spatial properties (no property realism concerning sensible qualities).

• • Even if such properties existed (in the form of powers or dispositions, as discussed below), simple mechanical causes could not transmit them to the rudimentary sensitive soul (no perceptual realism).

Thus, concerning Aristotle’s ‘proper sensible’ it no longer made sense to assert, ‘The perceiver is potentially what the perceptible object is already’ (DA 418a pp. 3–6).

2.2 The Biological Soul

The perception of sensible qualities was inexplainable, partly because the sensitive soul was conceived as a passive faculty. The Aristotelian tradition regarded perception as a naturalistic, bodily process, akin to growth, nutrition, reproduction, and local movement. As Sorabji puts it, ‘given Aristotle’s biological conception of the soul …, perception manifests life, not … consciousness’ (Sorabji, 1984, p. 68 ‒ emphasis added). This conception remained largely unchanged throughout the Middle Ages. In the perceptual psychology of this period, ‘there was no clear distinction between the physiological and the mental’ (Hatfield & Epstein, Reference Hatfield and Epstein1979, p. 370 ‒ italics added). Most medieval philosophers acknowledged, ‘Though organs are capable of sensation in virtue of being actualised by the soul, the story here is not fundamentally different from the story of our locomotive or digestive powers’ (Pasnau, Reference Pasnau and Marenbon2012, p. 496 ‒ my emphasis). Thus, proponents of medieval Aristotelian Scholasticism believed that ‘sensation is indisputably a biological process’ (op. cit.).

By adopting the Aristotelian sensitive soul as the instrument of perception, Galileo positioned himself within this tradition, implicitly acknowledging that the sensitive soul is a passive, biological capacity capable only of passively receiving external effects. It controls digestion and perception and both are to be seen as assimilation of a part of the environment: matter and form, respectively. Understood as the assimilation of substantial forms, this conceptualisation of perception was effective in explaining the perception of existing properties, in the absence of a substantial form, however, it did not work. Especially not in the case of non-existent properties, such as the colour of corpuscles. As an assimilative biological faculty, the sensitive soul did not have the constructive mental capacity to generate such perceptions from raw mechanical stimuli; therefore, the corpuscularists could not account for the perception of sensible qualities in an Aristotelian manner.

2.3 Disposition and Mind

Faced with these obstacles, philosophers of the generation after Galileo sought to modify the conceptual machinery of perception at two crucial points.

According to Aristotle, the substantial form of the perceived object fulfilled an active role in perception: it initiated the actualization of the potentiality of the sense organ. Similarly, after Galileo, several philosophers experimented with adding an active force to corpuscular objects to explain how we perceive secondary properties that, as such, do not exist in the objects (Wilson, Reference Wilson1992). Descartes, for instance, claimed that: ‘the properties in external objects to which we apply the terms light, colour, smell, taste, sound, heat and cold … are … simply various dispositions in those objects which make them able to set up various kinds of motions in our nerves’ (Descartes, Reference Descartes1644, p. 285 – my italics). Similarly, Boyle and Locke invoked ‘dispositions’ and ‘powers’ respectively, as active external entities accounting for such perceptions.

Locke posited that objects possess ‘powers to produce various sensations in us by their primary qualities’ (Locke, Reference Locke and Nidditch1689, p. II. x. ‒ original italics). At first glance, this formulation might seem not significantly different from Galileo’s view of the causal role of corpuscles. Thus, S. Shoemaker called it a merely semantic rather than a metaphysical difference that Locke spoke of a specific ‘power’ while Galileo did not (Shoemaker, Reference Shoemaker1990, p. 109). However, Locke makes a crucial distinction between the ‘idea’ in the mind (‘[w]hatsoever the mind perceives in itself, or is the immediate object of perception, thought, or understanding’) and the ‘quality’ in the object (‘the power to produce any idea in our mind’) (Locke, Reference Locke and Nidditch1689, p. II. viii ‒ my italics). Although his formulation is notoriously ambiguous, it is clear that he speaks of identifiable ‘qualities in the objects which produce ideas in us’ (Locke, Reference Locke and Nidditch1689, p. II. viii ‒ my emphasis). The qualities mentioned are not self-contained entities but specific constructions of the ‘bulk, figure, texture, and motion of their insensible parts’ (Locke, Reference Locke and Nidditch1689, p. II. x.). These ‘specific constructions’ are properties of objects that Locke labels as ‘secondary’ because they only become full-fledged qualities through an additional step, involving the contribution of the perceiver. As Locke says, ‘the power in fire to produce a new colour, or consistency in wax or clay by its primary qualities, is as much a quality in fire, as the power it has to produce in me a new idea or sensation of warmth or burning’ (Locke, Reference Locke and Nidditch1689, p. II. x. ‒ emphasis mine). Thus, he refers to inherent qualities of objects that manifest themselves through the perceptions of subjects. The powers and dispositions are seen as active forces capable of prompting perceptions of such properties in appropriate subjects. The perceptions that emerge in this way differ from those of primary properties in that they are not similar to their causes. So, these philosophers developed a dispositionalist explanation for how we perceive secondary properties.

This interpretation suggests that the primary and secondary properties differ in that the perceptions of the latter are not similar to their causes. However, from an ontological perspective, no essential distinction exists between the two: Locke ‘is quite clear that secondary qualities are genuinely in objects not in our minds’ (Cottingham, Reference Cottingham1989, p. 238 – my italics). If this is the case, then Shoemaker’s assertion is inaccurate since Galileo’s and Locke’s positions are metaphysically distinct. Locke assumes that the objects of perception include dispositional properties whereas Galileo does not (this will be discussed further). Consequently, Locke can elucidate the genesis of the perceptions of secondary properties by referring to objectively existing entities (powers), whereas Galileo cannot.

In fact, neither Locke nor Descartes provides a satisfactory account of this issue. The implantation of latent properties (powers or dispositions) in objects leaves the causal connection between them and the perceiving subject as elusive as in the case of the Aristotelian forms. It remains unclear how these dispositions ‘set up various kinds of motions in our nerves’ as Descartes asserts. Locke’s solution is no better. Although it is true that with the help of primary properties, the powers can initiate such movements, a further explanation of how the spatial properties can mechanically induce perceptions of colour, scent, etc., would still be needed.

The other solution to which post-Galilean philosophers turned offered an answer to this question. They replaced the passive, biological sensitive soul with an entity possessing the constitutive energy to produce sensible qualities from mechanical impacts. Thus, Descartes separated the biological and cognitive functions of the soul, stating that ‘the principle by which we are nourished is wholly distinct – different in kind, from that in virtue of which we think; [hence] … the term “soul”, when it is used to refer to both these principles, is ambiguous …’ (Descartes, Reference Descartes and Descartes1641a, p. 246 ‒ italics added). He could exempt the soul from its biological tasks because he considered the body a machine governed by mechanical principles therefore in no need for an animating soul. On the other hand, he claimed that cognitive procedures require a more intricate apparatus, the mind instead of the Aristotelian sensitive soul (Hardie, Reference Hardie1964, p. 54). As a result, he abandoned the long-established concept of the sensitive soul and retained only its incorporeal part, the mind, which had traditionally been seen as responsible for thinking and willing (but not perception). As an immaterial substance, the Cartesian mind was categorically distinct from bodily functions.Footnote ³ Its essence, being thought, allowed for the replacement of the assimilative soul with the constitutive mental capacity of the incorporeal mind.

This conceptual reorganization of the perceptual apparatus went with a relocation of the cognitive act of perception: Descartes redefined it as a mode of thinking, an immaterial mental state that emerges after the corporeal phase. In his view, ‘understanding, willing, imagining, having sensory perceptions and so on’ were all thoughts (Descartes, Reference Descartes and Descartes1641b, p. 124 – my italics). Perception took place in the mind that seemed to possess the psychological capacity to constitute perceptions of sensible qualities out of the mechanical effects of external things.

We see how the post-Galilean philosophy of perception sought to address the issue of perceiving non-existent secondary properties by implanting an active force in things and replacing the passive sensitive soul with an active mind. Galileo, however, did not adopt either of these solutions. This is why I contend that he is mistakenly associated with the tradition that distinguishes between two kinds of properties. Indeed, I argue that the retroactive extension of the Lockean ‘power’ and the Cartesian concept of ‘mind’ to his ideas rendered his distinction appear trivially wrong. Therefore, it can be surmised that following the unification of the Aristotelian sensitive soul with corpuscular metaphysics, Galileo could not incorporate sensible qualities into the domain of perceptions. In a sense, it was true, but this does not mean that his endeavour was a failure.

3.1 Heat, Pain, and Tickle

In the discussion on the nature of comets, Galileo and his adversaries debated whether comets were burning bodies ignited by friction that caused them to heat up. It was during this debate that Galileo introduced his distinction between two kinds of properties. He argued that comets could not be burning bodies because heat was not ‘a real attribute … which actually resides in the material’ (Galileo, Reference Galilei, Drake and O’Malley1623, p. 309). To clarify what he meant by this assertion, Galileo explained in his Dialogues Concerning Two New Sciences (Galileo, Reference Galilei, Crew and De Salvio1638) how fire particles liquefy a piece of solid metal.Footnote ⁴ He described a purely mechanical process: the fire particles penetrate the pores and fill the vacua among the corpuscles, thereby ceasing the quest of these corpuscles to occupy empty spaces. Thus, the force holding the particles together disappears, the structure of the metal collapses and the particles begin to move relative to each other, i.e., the solid metal changes to a liquid.

It is important to note that fire alters the state of matter without heat: it eliminates the empty spaces between corpuscles, thereby modifying the metal’s spatial structure (Galileo Reference Galilei, Crew and De Salvio1638, p. 19.). ‘Heat’ arises only when the same fire particles penetrate a sensitive body. In this case, they carry out the same type of physical rearrangements but this time the sensitive body feels their impacts as ‘pricks’, ‘divisions’ and ‘dissolutions’. Notably, the corpuscles of the sensitive body are not only rearranged by the fire particles:

The comparison of the two cases is intended to demonstrate that heat is not a property of external things, but an internal feeling induced by the sheer physical impacts on the sensitive body. As Galileo puts it, heat is a ‘sensation … produced in us by the passage and touch of the tiny corpuscles’ (Galileo, Reference Galilei, Drake and O’Malley1623, p. 312 ‒ italics added). He does ‘not believe at all that in addition to [geometrical properties] there is any other quality in the fire which is “heat”’ (Galileo, Reference Galilei, Drake and O’Malley1623, p. 312 – my italics]. Once again, in discussing the perception of the sensible quality of ‘heat’, no corresponding property is attributed to the penetrating corpuscles themselves.

One might argue that this is simply a theory of heat, which is either true or not, i.e., either there is heat in the world or there is not. However, Galileo’s notorious comparison between a marble statue and a living body further illuminates the matter. He juxtaposes again an inanimate with an animate entity claiming that when touched, the latter feels tickling while the former does not. This comparison is not merely a repetition of the former example. Galileo points out that contrary to heat, it would be absurd to assume that the sensation of tickling may have some objective counterpart in the world. Both sensations are elicited by ‘passage and touch’ but ‘anyone would seriously err who might wish to say that the hand had within itself, in addition to the properties of moving and touching, another faculty different from these’ (Galileo, Reference Galilei, Drake and O’Malley1623, p. 310 ‒ emphasis added). External objects have only physical properties that exert mechanical effects. Hence, regardless of how the impacting things differ in other respects (such as being a hand, a feather, or a piece of paper), they perform ‘what are inherently quite the same operations of moving and touching’ (ibid. ‒ emphasis added). Therefore, the sensation of tickling is not in the hand ‘in any way’ i.e., not even as a disposition or power: it ‘belongs entirely to us’, i.e., to the sensitive body (ibid. ‒ emphasis added).

The examples of ‘feeling hot’ and ‘tickling’ share common elements: both are induced by mechanical ‘passage and touch’, and the sensations, elicited in this manner, do not represent anything external but exist exclusively within the body of the perceiving subject. The juxtaposition of the marble statue and animate body not only demonstrates that the sensible quality of tickling cannot be a perception of any property of an external object but also reveals two other features of the perception of such properties.

1. 1. Different external objects (such as a hand, a piece of paper, or a feather) can produce the same sensation (tickling) in an animate body (Galileo, Reference Galilei, Drake and O’Malley1623, p. 309‒310).

2. 2. The same basic physical effect can prompt drastically different sensations

   1. a) depending on the part of the sensitive body they touch: it ‘excites [eccitare] in us an almost intolerable titillation while in other regions it is scarcely felt’ (Galileo Reference Galilei, Drake and O’Malley1623, p. 310 – my emphasis).

   2. b) Or, depending on the number and velocity of the penetrating particles, they elicit various sensations (pleasant warm, scalding, pain). (Galileo, Reference Galilei, Drake and O’Malley1623, p. 312).

This directs our attention to the causal mechanism that brings about these sensations.

3.2 Common Causes

The Aristotelian theory of perception hinged on the metaphysical thesis that the perceived object and the perceiving sense organ can share the same form. Therefore, the abolition of the metaphysical framework of matter and form rendered the Aristotelian explanation untenable. In line with the standard atomist doctrine, Galileo believed that perceptible things emit microscopic particles, which, upon colliding with the sense organs, cause physical alterations in them. These modifications could account for the perception of spatial properties but the mechanical causation seemed inadequate for explaining how perceptions of non-existing properties arise. Therefore, to explain how sensations of sensible qualities emerge, Galileo was compelled to develop a novel conception of causation. Although he proposed a solution, he did not elaborate on it, so it went largely unnoticed. He suggested that the collisions of corpuscles with the sense organs produce two simultaneous effects.

The sense organs undergo reconfiguration, leading to the perception of spatial properties. However, the collision of the corpuscles not only ‘set up various kinds of motions in our nerves’ as Descartes said. These are mechanical knocks and bangs that are also felt by the sensitive body. As a result, we become aware of two concurrent impacts: ‘[B]eing touched … [one] feels in addition to the general sense of touch another sensation… tickling’ (Galileo, Reference Galilei, Drake and O’Malley1623, p. 309 ‒ italics added).

The recognition that the perceptions of geometrical properties and sensible qualities have common causes liberated Galileo from the obligation of delineating a specific causal mechanism and distinct perceptual objects (powers, dispositions) for sensible qualities. The impinging corpuscles have spatial attributes only. These mechanically rearrange the corpuscles of the sensitive body, thereby bringing about perceptions of geometrical properties. The act of mechanical rearrangement constitutes a violent irritation of our sensitive body, which is naturally felt. These irritations give rise to the sensations that appear to us as sensible qualities. That is, they emerge as additional feelings accompanying the perceptions of spatial properties. Brought about by the same causes, sensible qualities are strongly correlated with perceptions of geometrical properties.

This conception of common causes provides an answer to how mechanical effects give rise to sensations of non-existent, non-spatial properties within us. Feeling pain and tickling are not special: all felt sensible qualities, from colours to odours, emerge in the same mechanical way. Fire particles themselves are not hot, but our body feels them as such when irritated by them. Similarly, the mechanical impacts of corpuscles can elicit sensations of colours, sounds, smells, and more, even though these properties do not exist in any material form. They are just physical stimulations, excitations that the passive, biological sensitive soul is capable of feeling. This is why Galileo declared he did not believe that ‘for exciting [eccitare] in us tastes, odours, and sounds there are required in external bodies anything but sizes, shapes, numbers, and slow or fast movements’ (Galileo, Reference Galilei, Drake and O’Malley1623, p. 311 ‒ italics added). All sensations of sensible qualities are of the pain type: variations in the number, shape, and speed of impacting corpuscles give rise to different sensible qualities through simple physical excitations. Whether mechanical irritations elicit visual, auditory, olfactory, or gustatory sensations depends on which part of the sensitive body is stimulated and the nature of the mechanical effects involved. ‘[T]he animate body [corpo animato] which receives these operations feels diverse sensations according to the various parts which are touched’ (Galileo, Reference Galilei, Drake and O’Malley1623, p. 309 – my emphasis). This resolves the problem of how we can sense properties that things do not possess. There is no need to postulate actual properties, hidden powers or dispositions in the objects or constructive capacities in a sophisticated mind, the same mechanical process provokes the perception of the spatial properties and the feelings of the accompanying sensible qualities. The sensitive soul alone can take care of both.

3.3 Internal Bodily Sensations

Being prompted by mechanical impacts of physical bodies that do not possess such properties, sensible qualities are not, in any sense, perceptions of the properties of external objects. The world consists of physical objects that have only geometric properties. These can be perceived. In contrast, felt sensible qualities are not representations of external properties but sensations of mechanical irritations of the animate body. Therefore, unlike later philosophers, Galileo did not need to introduce theoretical entities such as dispositions or powers to explain what we perceive when we become aware of such qualities.

He thought that the idea that external things have perceivable colours, smells, etc., in any sense was erroneous. Thus, he stated: ‘many qualities which we come to attribute to natural bodies, such as tastes, odours, colours … may be of similar and no more solid existence’ [than tickling] (Galileo, Reference Galilei, Drake and O’Malley1623, p. 310 – italics added). The mechanical irritations caused by the physical impacts of corpuscles on our sensitive body are typically understood as internal bodily sensations.

In addition to aligning with Galileo’s text, interpreting sensible qualities as internal bodily feelings rather than perceptions resolves the issue of his alleged perceptual scepticism. The claim that our senses keep flooding us with deceptive perceptual information making the world appear sonorous, colourful, fragrant and flavourful suggests that our senses mislead us. Such a conception is readily classified as ‘sceptical’. No wonder that, from the quasi-contemporary P. Bayle to the present day, numerous philosophers have regarded ‘the distinction between primary and secondary qualities … a strong impulse to scepticism’ (Smith & Charles 2017: xiv).

Although the sceptical understanding of Galileo’s distinction seems straightforward, there is a disconcerting fact. He never mentioned that a significant portion of our perceptions is systematically false or deceptive. Moreover, in his other writings, he takes great pains to show how our perceptions of primary properties can deceive us and how we can dispense with such deceptions. (Think of perceptual illusions of the size of heavenly bodies; the apparent immobility of the Earth, etc.) Given this, one cannot help but wonder why he allowed the vast and persistent illusion that things possess sensible qualities to go unchallenged and why the veracity of perception was paramount to him when it came to shape, movement or size.

I believe the reason for this is that Galileo recognised sensible qualities as internal bodily feelings rather than perceptions. As a result, he need not have been concerned with their accuracy in representing the world. Indeed, he believed that sensations of colours, sounds, and so on were exactly like pain or tickling – experiences that evidently do not represent physical reality and, therefore, lack veridicality conditions. Consequently, Galileo could maintain staunch anti-scepticism regarding the perception of the physical properties of external objects without having to consider sensible qualities that impart a deceptive appearance to the world. Lacking representative content, sensations of sensible qualities cannot misrepresent any property and therefore they are not illusions. Moreover, being causally connected to the physical world they are not hallucinations either. In fact, since they arise from mechanical irritations, sensible qualities are experienced despite our awareness that objects do not possess such qualities. In this sense, sensations of sensible qualities are incorrigible and their occurrence does not imply perceptual error. This elucidates why Galileo did not attempt to rectify these sensations.

In summary, Galileo’s recognition of sensible qualities as internal bodily sensations, devoid of veridicality conditions, justifies his treatment of them as cognitively irrelevant. Such sensations are not integrated into generalisations or abstractions about the world. Therefore, his indifference to the non-veridicality of these experiences did not indicate perceptual scepticism. Indeed, his consistent disregard for their acknowledged falsity indirectly confirms my interpretation that he did not consider these sensations as perceptions.

4.1 Ontological Difference

The prevalent misidentification of internal bodily sensations as perceptions obstructs a proper understanding of Galileo’s distinction. Arguments such as: ‘Matter can be conceived without magnetism, but no one is tempted to say that magnetismFootnote ⁶ is really in the mind, not the world’ (LoLordo, Reference LoLordo and Nolan2011, p. 71, – italics added) stem from this misunderstanding. Magnetism is not a metaphysically perceiver-dependent property; our capacity to conceive of objects both with and without it does not pertain to its existence. It persists regardless of whether it is perceived or not. In contrast, unfelt sensible qualities ‒ such as pain or tickling ‒ do not exist in the external world. The existence of such qualities is contingent upon the presence of a sensitive body capable of feeling them, rather than upon the comprehension of them by a mind. Thus, LoLordo’s argument could be rephrased as follows: ‘Matter can be conceived without its feeling a tickle, but tickling is really in the sensitive body, not the world’. The distinction between spatial properties and sensible qualities is ontological: unlike the former, the latter arise solely within the sensitive body.

This ontological distinction entails that ‘so far as pertains to the subject wherein they reside’, sensible qualities ‘are no more than mere names’ (Galileo, Reference Galilei, Drake and O’Malley1623, p. 309 ‒ my emphasis)Footnote ⁷. Consequently, Hamou’s interpretation is not supported by Galileo’s text. Hamou says that redness is ‘only a power to produce a perception of red or, as Galileo puts it, only a name for something in porphyry that is not red but makes porphyry look red’ (Hamou, Reference Hamou, Clarke and Wilson2011, p. 165 – underline is original, italics added). As we saw however, Galileo’s argument is not that the name refers to ‘something in porphyry’ but rather that there is nothing in porphyry, to which the word ‘red’ could be applied. As I tried to demonstrate, Galileo adhered to a nominalist ontology concerning sensible qualities, explicitly labelling it a ‘serious error’ to believe that there is anything in external things beyond their primary properties. Consequently, his expressions such as ‘only a name’ do not designate hidden external entities but reflect his view that language tricks us here. Names such as ‘red’, ‘smelly’ or ‘tasty’ refer to the internal sensations of the body not to something (exposed or hidden) inherent in external objects.

The categorisation of sensible qualities as internal bodily sensations also implies that ‘when the animate and sensitive body [corpo animato e sensitivo] is removed’ (Galileo, Reference Galilei, Drake and O’Malley1623, p. 310), sensible qualities cease to exist. Though the words ‘red’, ‘sweet’, etc., may remain in people’s memories, they cannot be projected onto external things in a scenario where no sensitive body is present. In such a situation a term like ‘heat’, for example, ‘remains nothing but a simple vocable’ (Galileo, Reference Galilei, Drake and O’Malley1623, p. 312 – italics added), mere sounds without reference. If Galileo had regarded sensible qualities as powers or dispositions inherent in objects, these would persist even if all living beings ceased to exist. Therefore, words like ‘red’ etc. would continue to refer to them. However, Galileo’s claim that with the disappearance of sensitive bodies, such names would lose their references suggests that they have always referred to bodily states, not powers or dispositions in external things.

Not only linguistic reference but also the conceivability of sensible qualities, presupposes bodily presence. The physical interaction between the world and our sensitive body is a prerequisite for becoming aware of such qualities. In contrast to the a priori comprehension of geometrical qualities, our ‘reason or imagination alone would perhaps never arrive at [sensible] qualities’ (Galileo, Reference Galilei, Drake and O’Malley1623, p. 309 – my italics). Once such a quality has been sensed, it can later be remembered or imagined, but direct bodily sensation is a precondition for our becoming aware of it in the first place. Consequently, sensible qualities cannot be ‘purely mental’ (LoLordo, Reference LoLordo and Nolan2011, p. 71) they are either presently felt by the body or exist as memories, or imaginations of previously felt bodily sensations.

Since sensible qualities are physically caused bodily sensations, the sensitive body plays an indispensable role in their existence. This is why Galileo repeatedly emphasizes that such qualities (unlike geometrical ones) would vanish from the world if our bodies ceased to exist: ‘tickling and titillation are nothing … when armpits and the skin around the nose are absent’ (Galileo, Reference Galilei, Drake and O’Malley1623, p. 311). Therefore, my conclusion is that Galileo’s real distinction is primarily ontological rather than epistemic. His aim is not simply to differentiate between existing properties that require a mental contribution to be recognised and those that do not. Rather, he wants to highlight the ontological disparity between properties that exist whether they are perceived or not, and qualities that come into being only in the act of corporeal perception. The former exist even if they are never perceived, whereas the latter ‘have no real existence except in ourselves’ (Galileo, Reference Galilei and Drake1632, p. 312).