For Aquinas, a sense receives the form of what it senses in a certain 'spiritual' way without matter. However, it receives the form 'with individuating material conditions,' and the object which is sensed by means of the received 'similitude' is a certain particular. The intellect can abstract from matter in different ways, all however requiring recourse to phantasms.
For Aristotle and St Thomas, sense perception, let alone intellection, is irreducibly different from other kinds of change. Clearly, we are not dealing with quantitative and local change, but there is also a very significant difference from ordinary qualitative change. Although all kinds of sense perception require in some way the use of corporeal organs, they are not merely changes between the basic dispositions of matter as, for example, hot and cold, dry and moist, rare and dense. Something hot can heat something else up, but it does not thereby cause that thing to feel heat. Aquinas distinguishes between immutatio naturalis and immutatio spiritualis (ST Ia Q.78 a.3). The first is when a form is received into that which is changed secundum esse naturale, or in a 'physical' manner. Such a physical mode of receiving a form occurs when form determines matter, and the matter receives that form. The example Aquinas gives of this is heat in something heated. In receiving the form of heat, the object thereby becomes hot. As opposed to this, a form may be received secundum esse spirituale. There is a real sense in which we 'receive' a colour when we see it. Aquinas says that the form of the colour is, in a way, in the pupil, but not in a way that makes that pupil coloured. Clearly immutatio spiritualis is irreducible to immutatio naturalis and cannot be explained by it alone. According to Aquinas, in sight, there is only immutatio spiritualis, but in other senses there is also immutatio naturalis. Nevertheless, that immutatio naturalis does not suffice to explain the sensation. If it did suffice for sensation, then all natural bodies would sense when so altered (ibid: si sola immutatio naturalis sufficeret ad sentiendum, omnia corpora naturalia sentirent dum alterantur). If the physical presence of something explained the cognition of that thing, then there would be no reason why, for example, fire would lack knowledge, since having fire, it would have awareness of fire (Q.84. a.2: si oporteret rem cognitam materialiter in cognoscente existere, nulla ratio esset quare res quae materialiter extra animam subsistunt, cognitione carerent, puta, si anima igne cognoscit ignem, et ignis etiam qui est extra animam, ignem cognosceret).
A constant refrain of St Thomas is that whatever is received is received according to the mode of the receiver. When it comes to inert and inanimate physical things, they can only receive forms that determine their matter in some physical way. Sense operations are operations of body and soul combined. The mere receptivity of matter for form cannot explain sensation, but, at the same time, sensation does require the use of corporeal organs. Each of the powers of sense which an animal has receive, secundum esse spirituale, the form of that aspect of something which they are determined to so receive. The power of sight, according to Aristotle and St Thomas, receives the form of colour. They say that it receives the form without its matter. For example, the power of sight receives the colour of gold without the gold (Q.84 a.1: forma sensibilis alio modo est in re quae est extra animam, et alio modo in sensu, qui suscipit formas sensibilium absque materia, sicut colorem auri sine auro). Also, sight sees the colour of, for example, an apple, without the smell of that apple (Q.85 a.3: visus enim videt colorem pomi sine eius odore).
St Thomas says that sense receives the form of the cognised thing sine materia but cum materialibus conditionibus or cum materialibus conditionibus individuantibus (Q.84 a.2). The colour as physically existing in the gold or the apple does not enter into the eye, but the colour seen is still seen 'with material conditions.' To understand this, we should note that the sense of sight is 'in act' when it receives the form of the thing sensed. This form is a certain similitudo of the sensible thing (Q.85 a.3: similitudo rei sensibilis est forma sensus in actu). Similitudo does not simply mean a mere symbol, which somehow represents the thing it signifies. Rather, it is the same forma which exists physically in the matter-form composite (the gold or the apple) and spiritually or intentionally in the sensus. The sensus has the ability to receive this form in a non-physical way. By 'same' forma, I mean that they are identical in species, not numerically. This is why this spiritually received form can be called a 'sensitive species' and can be called a similitudo of the forma physically existing in the matter-form composite. Second, although this impressed similitudo is that by which (id quo) the sensus senses its object, it is not that object itself (ibid: species sensibilis non est illud quod sentitur, sed magis id quo sensus sentit). The object of sensation is not the colour qua spiritually received as a similitudo by the sensus, but, rather, the colour, through that similitudo, is seen as being in the external object (the gold or the apple). This means that the similitudo or sensitive species does not just put the sense of sight in contact with 'yellow' or 'green' in general, but with that particular shade of 'yellow' or 'green' as it is seen in the particular external object. This seems to be at least part of what St Thomas means by the form being received cum materialibus conditionibus individuantibus(Q.84 a.2). This mode of receiving the form means that the object that is sensed will be a particular. Although the similitudo is received without the matter actually in the gold or the apple, the object of the sensitive power will be a form existing in a material body (Q.85 a.1: obiectum cuiuslibet sensitivae potentiae est forma prout in materia corporali existit. Et quia huiusmodi materia est individuationis principium, ideo omnis potentia sensitivae partis est cognoscitiva particularium tantum). Furthermore, although sight sees the colour of an apple without the smell, if it be asked where the colour is, which happens to be seen without the smell, that colour will be in no other place than in the apple. The sight receives the similitudo of the colour, through which it sees the colour in the apple, but it does not receive the similitudo of the smell in that apple, and so is not perceptive of the smell in that apple (Q.85 a.3: visus enim videt colorem pomi sine eius odore. Si ergo quaeratur ubi sit color qui videtur sine odore manifestum est quod color qui videtur, non est nisi in pomo; sed quod sit sine odore perceptus, hoc accidit ei ex parte visus, inquantum in visu est similitudo coloris et non odoris).
The intellect also receives according to its mode of receiving. The intellect is not a power of any bodily organ, and so is an immaterial power in a unique sense. Bodily things are particular, mobile, and contingent. How then can the intellect know anything about them? If it could not, then the whole of natural philosophy would be impossible. Aquinas thinks that the intellect somehow knows these things with an immaterial, universal, and necessary knowledge (Q.84 a.1: anima per intellectum cognoscit corpora cognitione immateriali, universali et necessaria). The reason why this is possible is because we can still identify something necessary, stable, universal, and formal about even contingent, mobile, particular, and material things. For example, all motion supposes something permanent. A substance persists in accidental change, and even in substantial change, matter persists (ibid: omnis motus supponit aliquid immobile, cum enim transmutatio fit secundum qualitatem, remanet substantia immobilis; et cum transmutatur forma substantialis, remanet materia immobilis). Another example is that, although Socrates does not have to sit, it is necessary that whenever he does sit, he must be in a certain place (Socrates etsi non semper sedeat, tamen immobiliter est verum quod, quandocumque sedet, in uno loco manet). Therefore, even about mobile things, 'immobile knowledge' is possible (et propter hoc nihil prohibet de rebus mobilibus immobilem scientiam habere). Furthermore, natural philosophy deals not just with matter but also with form, and not just with individual matter but with 'common matter.' Form gives determination and stability of identity to a particular substance. And any sort of physical thing must be composed of the relevant form and some matter, but not necessarily this or that individual matter. Therefore, a sort of thing (an essence) can be considered apart from individualising and contingent conditions.
This brings us to the question of abstraction. In response to the second objection in ST Ia Q.85 a.1, St Thomas outlines three grades or levels of abstraction. The most basic kind is the abstraction of the 'species of a natural thing' from individual sensible matter. To merely sense something or imagine something through a phantasm, we are restricted to a particular. What the intellect can do is abstract from the phantasm the general species rei naturalis from an image of just a particular individual example of one of that species. Over and above 'this flesh' and 'this bones,' the agent intellect can abstract 'flesh' and 'bones' in general. The former is what is called materia signata vel individualis and the latter is called materia communis. This is achieved by a general idea of flesh and bones. However, this general idea cannot be considered without reverting back to the phantasms that provide examples of it. We cannot understand the essence of water without making use of a particular image of water, even though, through our intellect, we are able to consider that which is common to any water aside from the individuating conditions of this particular image. Therefore, the intellect abstracts intelligible species a phantasmatibus, which allows it to understand the natures of things universally, but it understands those natures in phantasmatibus (intellectus noster et abstrahit species intelligibiles a phantasmatibus, inquantum considerat naturas rerum in universali; et tamen intelligit eas in phantasmatibus, quia non potest intelligere etiam ea quorum species abstrahit, nisi convertendo se ad phantasmata).
The second grade of abstraction is mathematical abstraction, where we consider quantity over and above any qualities such a quantified thing would exhibit in nature. Even here, though, we still require phantasms. Although, we understand triangularity regardless of whether a triangle is red, blue, or whatever colour, we need to utilise an image of triangle which does have a certain colour.
Finally, the intellect can abstract from all matter, in considering those things which are able to exist apart from any matter (quae etiam esse possunt absque omni materia). However, the human intellect in its present condition can only think about these things by comparison and analogy with material things. Therefore, phantasms of those material things are required.
Different levels of immateriality correspond to different levels of cognition. Sensation is not wholly a material process, but it does require a corporeal organ and terminates in a particular existing in a material individual. The intellect can know about the universal nature of material things, quantity abstracted from any quality which it must co-exist with in a physical object, and immaterial things by comparison and analogy with material things. All of those three ways of knowing require the utilisation of phantasms.
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