Home Economics Aquinas’s theory of perception: an analytic reconstruction
Colour as Essentially Visible
Aquinas considers colour as being essentially visible: ‘First of all, then, he says that, colour being visible, it is visible of itself, for colour as such is essentially visible’ (Commentary on the Soul, no. 400). An immediate problem for any philosopher concerned with philosophical analysis: what does Aquinas mean by the use of the term ‘essentially’? If the term is to have any value, Aquinas must accept some kind of necessary relation between colour and visible. It appears, however, that this necessary proposition cannot be reduced to an analytic a priori definition. This follows because Aquinas has already entertained the possibility that there are visible objects that are perceived in the dark. These ‘other’ objects of sight, which have to be visible in some sense, cannot be coloured, because colour is not manifested without some form of light. Therefore, it appears that for Aquinas, ‘being visible’ and ‘being coloured’ are neither related analytically nor coextensive.
Aquinas pursues this point through an attempt to explicate the different senses in which ‘essentially’ can be taken:
‘Essentially’ [per se] is said in two ways.
In one way, when the predicate of a proposition falls within the definition of the subject, e.g. ‘Human is an animal’; for animal enters into the definition of human. And since that which falls within the definition of anything is in some way the cause of it, in cases such as these the predicate is said to be the cause of the subject.
In another way, on the contrary, when the subject of the proposition falls within the definition of the predicate, as when it is said that a nose is snub or a number is even; for snubness is nothing but a quality of a nose, and evenness of a number which can be halved. And in these cases the subject is a cause of the predicate. (Commentary on the Soul, no. 401; emphasis added)
Aquinas continues this analysis:
Now colour is essentially visible in this second manner, not in the first. For visibility is a quality, as being snub is a quality of a nose. And this is why he says that colour is visible ‘essentially’ but not ‘by definition’; that is to say, not because visibility is placed in its definition, but because it possessed of itself the reason why it should be visible, as a subject possesses in itself the reason for its own peculiar qualities. (no. 402)
These are interesting passages in which Aquinas distinguishes two categories of necessary propositions. Using a venue from later philosophy, it appears that Aquinas attempts to distinguish synthetic necessary propositions from analytic necessary propositions. In order to make this point clear, he provides two distinctions regarding propositions. His remarks regarding an essential relation between subject and predicate are twofold. First, an essential relation holds if the predicate ‘falls within the definition of the subject’. Aquinas gives as an example the following propositions: ‘Human is an animal.’ In this case, it appears that Aquinas claims that ‘animal’ is a part of the definition of ‘human’. Accordingly, this is an analytic a priori definition.
In the second case, Aquinas claims that the ‘subject falls within the definition of the predicate’. As examples of such a relation between subject and predicate he offers the following propositions: ‘Nose is snub’ and ‘Number is even. In these cases, the predicate is not a part of the definition of the subject; rather it is a quality of a number, which can be halved, or of a nose, which is of a certain shape. In this discussion, he considers the issue of necessary qualities. Snub and evenness are related necessarily to their subjects. Both the above cases consider the concept of necessity. The first case is an example of an analytic a priori definition. In other words, ‘animal’ as a predicate is part of the definition of what a human being is. Accordingly, the property of ‘being an animal’ is a necessary condition for ‘being a human’. This aspect of necessary condition is what Aquinas refers to when he asserts that the predicate is the cause of the subject. This conception of cause is not an efficient cause, but in the Aristotelian analysis it is a formal cause. In the final analysis, therefore, part of the definition of human is the predicate ‘animal’.
In the second case, the predicate is not a part of the definition of the subject. Rather, it is a quality of the subject. However, it is not an accidental property but an empirically necessary property. Certain subjects are so structured that they have the property in question and only those subjects possess that property. Aquinas brings out this aspect when he claims that ‘a subject possesses in itself the reason for its own peculiar qualities. Not just any quality is considered here, but only a quality that is ‘peculiar’ to the subject. This is, therefore, an empirically necessary property. Aquinas does not claim that a quality must belong to a subject essentially. Rather, he claims that certain qualities inhere with certain subjects and only those subjects. His example of ‘number is even’ illustrates this point, when he suggests that evenness is a quality by which a number can be halved. This quality is only what it is, however, because the subject has certain other qualities or dispositions. In the end, such a property is related necessarily to the subject because the subject is so structured that it is never found without that particular quality. 
In summary, therefore, regarding colour, the proposition ‘colour is visible’ is not an analytic a priori definition. Rather, ‘being visible’ is an empirically necessary dispositional property of colour. In other words, being visible is a necessary property or quality of colour. However, from the example found in the passage from the Commentary concerning ‘glow-worms and certain fungi on oak trees’, not everything that is visible is coloured or seen by means of its colour. Accordingly, the two senses of essential predicates mentioned in the passage under consideration are distinguishable into the categories of formally necessary (i.e. a formal cause) and empirically necessary propositions. In fine, therefore, ‘visible’ is related to ‘colour’ by means of an empirically necessary relation of predication and not through a formal definition of colour.11
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