Aquinas and Causal Theories of Perception

In discussing Aquinas and direct realism, his position on a causal theory of perception requires explanation. Succinctly put, a causal theory of perception is a version of externalism in which what a knower perceives has some direct or appropriate causal relation with the object that is known. In much history of philosophy, a connection exists between representative or indirect realism and causal theories of perception. Locke is a prime example; the Greek atomists with their account of idola adopted this position. Early twentieth-century sense data philosophers are contemporary examples. The epistemological assumption since the rise of the new science suggests that if we are to have knowledge, there is a need to posit intervening entities of some variety that stand between our minds and the objects existing in the external world. However, one need not be a representationalist and hold a causal theory of perception. This depends on the kind of causal relation that exists between the object of perception and the content of the idea. Although Aquinas provides a causal theory of perception, nonetheless he maintains a position of direct realism. Given his faculty psychology of dispositional properties as knowing powers, he argues that the objects in the world are potentially sensible and intelligible. Hence, there is no intervening mental entity between the mind and the world. This isomorphism is captured by Aquinas’s Latin propositions Sensus in actu est sensible in actu, and Intellectus in actu est intelligible in actu. He transcends the limits of efficient causality normally connected with both the philosophers of the new science and modern empiricists. Following Aristotle, Aquinas opts for a theory of formal causality and for efficient causality. The analysis of causality in his theory of perception cries out for clarification.[1]

Representative realism, as noted above, argues that a perceiver is never aware directly of a physical object. There are at least three different rationales from which philosophers argue for representative realism. These apply especially to Descartes and Locke:[2]

  • (a) a causal theory of perception;
  • (b) the problem of error and illusion;
  • (c) the scientific distinction between ‘the real’ and ‘the apparent’.

Representative realism in principle raises fundamental questions about the veridical nature of sensation and perception. If representative realism is correct, the following three questions might be asked:

  • (a) What is the ontological status of the representative entities? In other words, what is the status of the ‘idea’ or ‘sense datum’?
  • (b) What does ‘represent’ mean? Put differently, what is the ‘logic’ of the proposition ‘X represents Y?’
  • (c) How does one know that the representative or representing entity represents anything at all? Historically, this was Malebranche’s worry concerning Descartes’s version of representationalism. Berkeley’s concerns about the adequacy of causal explanations in Locke are similar in structure.

The causal theory of perception is linked with representative theories of perception in the following way: there is a causal bombardment of the sense faculties with material entities producing a reaction, which is the object of knowledge. An illustrative model would be atomism, in which the ‘idola were given off by the physical object and then produced an ensuing reaction with the sense faculties. With Aquinas, therefore, it remains to be established how one can have a causal theory of perception that does not entail representative realism. This requires a different analysis of cause from that provided by the early modern philosophers, especially Locke and Descartes. A distinction between efficient and formal cause is a necessary condition for this explication.[3] The rise of the new science brought about a dichotomy between what is ‘real’ and what

‘appears’ to a perceiver. Simply put, this is the ‘microscopic’ versus ‘macroscopic’ world-view problem. This scientific world-view forced a huge theoretical chasm between the medieval philosophers and their early modern counterparts. That Aquinas was not concerned about this scientific problem is obvious—he lived some four centuries before the scientific revolution.

Given the three issues whose consideration might result in representationalism, only the analysis of a causal theory of perception requires in-depth elucidation. Like many mid-twentieth-century ordinary-language philosophers, Aquinas spent little time considering the possibility that the problems of error and illusion are viable philosophical issues. Lastly, since he lived well before the scientific revolution, the concern over the ‘appearance/reality’ distinction consequent upon the primary/secondary quality distinction has no bearing on his theory. It follows, therefore, that his view of cause will be a salient issue in distinguishing his theory from the representative realists of the seventeenth century.

In discussing the causal theory of perception as championed by Locke and other mechanists, Berkeley put the problem thus: how could an object produce a representation of itself? Put differently, how can an object communicate knowledge of itself? The causal mode characteristic of these seventeenth-century discussions, however, was one of ‘push-pull’—i.e. there existed a causal chain of impulse. One might suggest that Locke and Descartes combined an Aristotelian ‘effect, which would be the idea produced, with a ‘new science’ account of cause. At issue here, obviously, is an analysis of the medieval view of cause. The medievals followed Aristotle in using a ‘qualitative’ view of causality, with emphasis on the distinctions between a formal cause and an efficient cause.

Even though some historians of philosophy suggest that the causal theory of perception alone is sufficient to generate representative realism, this is predicated upon the view that causal interaction occurs only on the impulse or efficient-cause model. In the new science view of sensation and perception, human perceivers have bodies— collection of atoms—among other bodies. Sensation and perception are the result of a causal change. Moreover, when an idea is produced, this is not a special case of causation. For many medievals, to the contrary, causality exercised in knowledge situations is a special case, and ‘intentionality’ becomes important for understanding medieval accounts of the philosophy of mind. On a logical reconstructionist model, for the medievals, the notion of intentionality is a primitive, explanatorily basic principle. In this case, ‘primitive’ does not mean that no analysis is needed. It is more like an intrinsic, non-acquired, irreducible capacity or power of a human person. This intentional capacity needs to be developed and actuated on the cognitive level; nonetheless, it is primitive in the sense that it is a constitutive feature of a human person. It is rooted in the substantial form of human nature.

For the early moderns, this causal change is not a special case of causation; hence, intentionality or mental acts are reducible to a mechanistic analysis; a mental act is like ‘melting wax’—i.e. the logic of the two events is the same. Given this view, the visual field is treated as an effect, which is the end of a mechanistic causal change. What is produced is the object of perception, not the mental act of awareness. This is, in effect, the ‘inner theatre’ view of perception.[4]

Although Aquinas is involved in a causal theory of perception, his theory does not generate representationalism. First of all, as noted in Chapter 2, Aquinas adopts a special thesis of intentionality. Secondly, his view of cause involving awareness will be an analysis in terms of a qualitative model and not a quantitative or mechanistic model. Thirdly, the mental act will always be that ‘by means of which’ (a quo) we acquire an awareness of the world around us in normal awareness situations; it is not the direct object (id quod) of sensation or perception. Lastly, causality is discussed in Aquinas as a formal cause, which is not reducible to an efficient cause. Modern philosophy adopted principally the paradigm of efficient causality. Although Aquinas adopts a causal theory of perception, his analysis of cause in principle avoids the pitfalls of representative realism. Like Putnam, Aquinas does not adopt the ‘inner theatre’ approach to perception.[5]

In De Veritate, Aquinas comments on his position that to know is to have the form of another not naturally but intentionally. This is, for Aquinas, the fundamental principle of knowing. In De Veritate one finds him writing that ‘knowing in us is the stamping of things on our minds’ (De Veritate, q. 2 a. 1 ad 2). Thomas develops this position: ‘Human knowers have actual sensations or actual knowledge (understanding) only because our senses or our intellects are informed by the species or likeness (similitude) of the sensible or intelligible object’ (Summa Theologiae, Ia q. 14 a. 2). In his Aristotelian Commentary on knowledge, Aquinas writes: ‘Accordingly, a sense receives form without matter, the form having, in the sense, a different mode of being from that which it has in the object sensed. In the latter, it is a material mode of being (esse naturale), but in the sense, a cognitional and immaterial mode (esse intentionale)’ (Commentary on the Soul, no. 553). Aquinas continues: ‘for the sense is assimilated to the sensible object in point of form, not in point of the disposition of the matter’ (no. 554).

  • [1] There are more distinctions regarding causal theories of perception than are discussed here.
  • [2] The author is indebted to Alan Hausman, who first suggested these distinctions.
  • [3] Historically, it appears that the problem of error and illusion did not influence Locke but did concernDescartes, especially in the ruminations central to the First Meditation. Nonetheless, error and illusionhave influenced philosophers in the early and mid-20th c. who have argued for sense data positions. Forexample, in his Foundations of Empirical Knowledge, Ayer appeared to use error and illusion as aporialeading to representative realism. Like Aristotle, Aquinas seems not to have been bothered by the problemsof error and illusion. In discussing these issues, Aquinas resembles ordinary language philosophers in themid-20th c. dismissing the possibility of sense data. Aquinas rejects Descartes’s dream problem almostout of hand. His appeal is to common-sense notions of consistency and coherence; he seems to be saying:‘A dream image just does not fit together, and if you think it does, you’d better think again!’
  • [4] Sorabji appears to argue that Aristotle’s position on intentionality is reducible to a form of mechanism.See Richard Sorabji, ‘Intentionality and Physiological Processes: Aristotle’s Theory of Sense-Perception, inMartha Nussbaum and Amelie Rorty (eds), Essays on Aristotle’s De Anima (Oxford: Clarendon Press,1992), 195-226.
  • [5] The complete analysis of how this causal theory is possible will be explained later.
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