Aquinas and Empiricism From Aquinas to Brentano and Beyond
The preceding chapter provided a detailed analysis of the principles of intentionality, which serve as the necessary conditions or presuppositions that Aquinas used in developing his philosophy of mind. The present chapter continues this discussion with six additional presuppositions necessary for understanding Aquinas’s explanation of how knowledge is possible.
This general history of philosophy locates the place Aquinas holds in philosophy-of- mind discussions, both traditional and contemporary.
Aquinas as an Empiricist
In two senses of the term, Thomas can be considered an empiricist. First, he refused to admit into his ontology any subsistent entities which would serve as objects of knowledge beyond the individually existing concreta of the physical world. Using terminology from medieval philosophy, Aquinas denied the existence of universalia ante rem, which is a version of classical Platonism. Ontologies, therefore, which admit subsistent entities, be they Platonic Forms, Moorean Propositions, Meinongean Inexistent Objects, or objects in Lewis’s or Plantinga’s possible worlds, are in opposition to Aquinas’s philosophy. He opts for a structured mental act, which is opposed to the diaphanous mental act of Plato, Moore, Russell, Meinong, and others; in this way, 1
Aquinas undercuts the philosophy-of-mind requirement for positing subsistent objects, and remains an empiricist. In this first sense, he is an empiricist because his ontological realism lacks subsistent entities determining essences beyond the space-time realm. This position articulated by Thomas is a denial of classical Platonism.
Secondly, Aquinas is classified as an empiricist because he adopts the epistemological maxim ‘Nihil est in intellectu quod non prius fuerit in sensu. The manner in which Aquinas accepts this proposition must be qualified, however. He asserts a more Leibnizean than Lockean or Berkeleyan interpretation of this proposition. Aquinas would be in agreement with Leibniz’s characterization of Locke’s maxim: ‘Nihil est in intellectu quod non prius fuerit in sensu, nisi intellectus ipse. In other words, Aquinas is not an empiricist in any sense structurally identical to or coextensive with much British or American empiricism. This claim will become apparent later when the structured acts of awareness of the vis cogitativa and the intellectus agens are discussed. Both of these acts of awareness provide an esse intentionale for a kind of object. It is by means of a structured mental act of both the vis cogitativa and the intellectus agens that Aquinas transcends the limits common to British and American empiricism. The vis cogitativa provides an awareness of the individual concretum as a subsistent particular; this is more than the bundle of sensations common to the empiricist model of perception whose associationist techniques have evolved from Locke and Berkeley through Hume to Mill and on into the twentieth century. By means of a sophisticated account of intentiones non sensatae, Aquinas provides the machinery for an awareness of individual concreta. In addition, it is through the structured mental act of the intellectus agens abstracting the species intelligibilis that Aquinas provides for the acquisition of knowledge with the intellectus possibilis of essential or sortal properties. What is important to realize is that Aquinas, while an empiricist in a substantive sense, is also to a certain degree a rationalist. Through the means of a structured mental act, he transcends the limits of classical empiricism yet avoids the pitfalls of Cartesian innate ideas. Nonetheless, the empiricist tenor of his philosophy of mind entails that all knowledge must begin in sense experience.
Issues in realism and externalism, together with the connection with empiricism, will become more clear as this analysis of Aquinas’s works unfolds. In his Compendium of Theology, he considers his empiricism:
Nonetheless, we should not lose sight of the fact that the human intellect is indebted to the sense powers for the origin of its knowledge. This is why intellectual knowledge is thrown into confusion when the soul’s faculties of phantasm [phantasia], imagination or memory are impaired. On the other hand, when these powers are in good order, intellectual apprehension becomes more efficient. (Compendium of Theology, pt I, ch. 122; emphasis added)
Aquinas continues with this theme: ‘an injury to an organ of the body may indirectly weaken the intellect, insofar as the activity of the intellect presupposes sensation (Commentary on the Soul, no. 688). These passages indicate the dependence of the intellect on the senses. This, of course, is a classical expression of empiricism. This dependence of the intellect on the senses is threefold:
All three propositions indicate the manner in which Aquinas is an empiricist in his philosophy of mind and in his epistemology.
While accepting a modified form of empiricism, nonetheless Aquinas accepts a semblance of Platonic duality regarding knowledge:
The following two texts indicate this twofold division regarding knowledge:
Aristotle discriminates between actual sensation and thinking; and he believes the first reason for distinguishing these activities is the difference between their objects, i.e. the sense-objects and the intelligible objects, which are attained by actual sensation and actual thinking respectively. (Commentary on the Soul, no. 375)
Aquinas continues by considering the objects of the two categories of knowledge:
The sense objects that actualize sensitive activities, the visible, the audible, etc., exist outside the soul; the reason being that actual sensation attains to the individual things, which exist externally outside the mind. Rational knowledge, on the other hand, is of universals, which exist
somehow within the soul. Thus, it is clear that the person who already has scientific knowledge about certain things does not need to seek such things outside of himself; the person already possesses them inwardly. Hence, one is able, unless prevented by some incidental cause, to reflect on them whenever one pleases. But a person cannot sense whatever one pleases. Because sense objects are not possessed inwardly, one is forced to receive them from outside. (Commentary on the Soul, no. 375; emphasis added)
Another difference between sense knowledge and intellectual knowledge concerns the necessity of a bodily organ: ‘Now the difference between intellectual and sensitive awareness is that the latter is corporeal. Sensation cannot occur apart from the mental act of a bodily organ. Understanding, on the other hand, as we shall prove later, does not take place by means of such an organ’ (Commentary on the Soul, #622). Using this distinction between sense knowledge and intellectual knowledge, Aquinas reiterates the Platonic distinction between sense knowledge or opinion and intellectual understanding or science. Like Plato, the former deals with human awareness of the particular things and sensible qualities of the world, while the latter treats knowledge of essences and universals.