Right, that’s an excellent question. There is also a lot of diversity in answers here across traditions, although I think we can also identify some strong isomorphisms between them.
A difficulty here is that the contemporary reception of these has been hamstrung by the fact that most philosophical pedagogy (including many popular survey texts, such as Durant’s) simply skip straight from Aristotle to Descartes. Yet Aristotle himself only provides a basic outline for what would become the distinction between the discursive faculty of dianoia (ratio) and the receptive faculty of noesis (intellectus), which isn’t given a fully fleshed out phenomenological grounding until the Middle Platonists.
Anyhow, it’s worth keeping in mind that on the discursive-only picture:
-chains of justification must terminate,
-inference presupposes validity,
-rules presuppose correctness conditions.
Thus, such accounts will always rely on something non-discursively grounded, even if these are said to be “just what we do,” or are grounded in voluntarism, brute fact, etc. The question, then, is whether that ground is:
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intelligible and normatively binding, or;
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brute, voluntaristic, or merely habitual
The demand for discursive justification of noesis assumes that only discursive justification counts. Yet discursive reason itself depends on a prior grasp of principles, unities, and necessities.
With that in mind, we have to consider that an elucidation of noesis/intellectus (or something like it) is not going to be able to work according to the presuppositions of dyadic mechanism. I think modern philosophy has picked over this issue quiet enough to show that if you reduce causality to temporally ordered mechanism, it becomes mere brute patterning (something Hume saw so well).
So, what is needed is something of a metaphysics of knowledge, which will involve an explication of causality. That’s a tall order for a single post (but I’ll share two examples I like below). For now, I’ll just focus on the most common concern, which is that noesis must reduce to “you just know.”
I think this misconception comes out of misreadings of Aristotle in the lends of modern fondationalism. Noesis doesn’t preclude error, not least because our grasp of the actuality of even finite beings is not absolute in any of these traditions. As Saint Thomas puts it, all the efforts of the human intellect will never exhaust the essence of a single fly (Saint Theresa of Avila makes a similar case with water, and Eriugena opens the Periphyseon with this point). To know a thing exhaustively is to know its entire context, and to know its causes and principles, which brings you out on an ascending chain of principles and a horizontal chain of contexts—presumably to a First Principle. But this sort of knowledge was considered impossible for man.
Hence, a grasp of form/actuality/Li/logoi, etc. need not rule out error.
However , crucially, truth is distinguished from error not by further discursive derivation, but by whether or not what is grasped really does function as a principle—an intelligible “one” that governs the many (e.g., the way the principle of lift explains flight in airplanes, insects, birds, etc.).
Once dianoia and noesis are seen as complementary rather than rival faculties, the standard objections re error lose their force. Discerning truth from error and wisdom from folly involves both.
For instance, understanding the principles of arithmetic is not simply predicting how one will go on, nor forming a hypothesis about future behavior,nor encoding a decision procedure, but rather grasping the form that governs the many.
Intelligibility is immanent to the object of understanding. Noesis receives it. Dianoia articulates and applies it. But the articulation is contentless unless the terms are understood. Error can occur under the aspect of reception because terms can be clear or unclear, which is why knowing “all snakes are reptiles” is true is not knowledge of the sentences content if one does not understand what a snake or reptile is (the terms, which are not of themselves true or false).
Here is one example from Nathan Lyon’s Signs in the Dust. Robert Sokolowski’s The Phenomenology of the Human Person also does a really good job on this question and brings in a lot of helpful insights from contemporary phenomenology and philosophy of language.
The particular expression of intentional existence—intentional species [i.e., form] existing in a material medium between cogniser and cognised thing— will be our focus…
In order to retrieve this aspect of Aquinas’ thought today we must reformulate his medieval understanding of species transmission and reception in the terms of modern physics and physiology.11 On the modern picture organisms receive information from the environment in the form of what we can describe roughly as energy and chemical patterns. 12 These patterns are detected by particular senses: electromagnetic radiation = vision, mechanical energy = touch, sound waves = hearing, olfactory and gustatory chemicals = smell and taste.13 When they impinge on an appropriate sensory organ, these patterns are transformed (‘transduced’ is the technical term) into signals (neuronal ‘action potentials’) in the nervous system, and then delivered to the brain and processed. To illustrate, suppose you walk into a clearing in the bush and see a eucalyptus tree on the far side. Your perception of the eucalypt is effected by means of ambient light—that is, ambient electromagnetic energy—in the environment bouncing off the tree and taking on a new pattern of organisation. The different chemical structure of the leaves, the bark, and the sap reflect certain wavelengths of light and not others; this selective reflection modifies the structure of the energy as it bounces off the tree, and this patterned structure is perceived by your eye and brain as colour…
These energy and chemical patterns revealed by modern empirical science are the place that we should locate Aquinas’ sensory species today.14 The patterns are physical structures in physical media, but they are also the locus of intentional species, because their structure is determined by the structure of the real things that cause them. The patterns thus have a representational character in the sense that they disperse a representative form of the thing into the surrounding media. In Thomistic perception, therefore, the form of the tree does not ‘teleport’ into your mind; it is communicated through normal physical mechanisms as a pattern of physical matter and energy.
The interpretation of intentions in the medium I am suggesting here is in keeping with a number of recent readers of Aquinas who construe his notion of extra-mental species as information communicated by physical means.18 Eleonore Stump notes that ‘what Aquinas refers to as the spiritual reception of an immaterial form . . . is what we are more likely to call encoded information’, as when a street map represents a city or DNA represents a protein. 19… Gyula Klima argues that ‘for Aquinas, intentionality or aboutness is the property of any form of information carried by anything about anything’, so that ‘ordinary causal processes, besides producing their ordinary physical effects according to the ordinary laws of nature, at the same time serve to transfer information about the causes of these processes in a natural system of encoding’.22
The upshot of this reading of Aquinas is that intentional being is in play even in situations where there is not a thinking, perceiving, or even sensing subject present. The phenomenon of representation which is characteristic of knowledge can thus occur in any physical media and between any existing thing, including inanimate things, because for Aquinas the domain of the intentional is not limited to mind or even to life, but includes to some degree even inanimate corporeality.
This interpretation of intentions in the medium in terms of information can be reformulated in terms of the semiotics we have retrieved from Aquinas, Cusa, and Poinsot to produce an account of signs in the medium. On this analysis, Aquinas’ intentions in the medium, which are embeded chemical patterns diffused through environments, are signs. More precisely, these patterns are sign-vehicles that refer to signifieds, namely the real things (like eucalyptus trees) that have patterned the sign-vehicles in ways that reflect their physical form.24 It is through these semiotic patterns that the form of real things is communicated intentionally through inanimate media. This is the way that we can understand, for example, Cusa’s observation that if sensation is to occur ‘between the perceptible object and the senses there must be a medium through which the object can replicate a form [speciem] of itself, or a sign [signum] of itself ’ (Comp. 4.8). This process of sensory semiosis proceeds on my analysis through the intentional replication of real things in energy and chemical sign-patterns, which are dispersed around the inanimate media of physical environments.