Augustinian philosophy has three primary principles: interiority, participation, and immutability. For now, I would like to highlight the second principle of participation and the theory of divine illumination flowing out of it; and to do this I will quote Johannes Quasten’s explanation at length,
“2. The second principle which enters into the essential nucleus of Augustine’s philosophy is that of participation, which is also a well known doctrine. Follow the De mor. eccl. cath. 2, 4, 6 the principle can be stated as follows: every good is either good by its nature and essence or is good by participation: in the first case it is the Highest Good, in the second it is a limited good. The same principle, with explicit reference to creation, can also be stated in another way: ‘Every good either is God or proceeds from God’ (De v. rel. 18, 35). But since in the unity of the human spirit life takes on a triple form, i.e., being, knowing and loving, so does the principle of participation take on the same form and thus becomes the participation in being, in truth and in love. From this triple form of participation there arises the notion, which is so frequent in Augustine, of God as the cause of being, the light of understanding and the source of love (De civ. Dei 8, 4; 8, 10, 2). There also arises the three-fold division of philosophy into natural, rational and moral (De civ. Dei 2, 7; 8, 4) and, finally, the essential solution of each of these three parts in creation, illumination and beatitude which are, then, the three modes of expressing the one doctrine of participation.”
As indicated by the above reference, illumination is part of the Augustinian philosophical principle of participation. Quasten continues to clearly explain Augustine’s theory of divine illumination in the following passages,
“The second fundamental solution of Augustinian philosophy which is closely bound to the first is the theory of illumination. It is another aspect – the second – of the doctrine of participation (cf. p. 408). ‘Our illumination is a participation in the Word, that is, in that life which is the Light of men’ (De Trin. 4, 2, 4). In order to facilitate the understanding of this theory, which has proved to be a constant problem for interpreters, some of its essential points will be presented here.
Since it is an aspect of participation, illumination cannot be understood apart from the doctrine: if God is the source of being, He is also the light of understanding. He is, therefore, the interior teacher who instructs man in the truth (De mag. 12, 39-14, 46), He is ‘the sun’ of the soul (Solil. 1, 8, 15) ‘in which and from which and through which all intelligible things shine in an intelligible way on the soul who understands’ (ibid., 1, 1, 3). It is ‘in the Truth itself…in God that we see the immutable ideal of justice according to which we judge it is necessary to live’ (De Trin. 8, 9, 13). Indeed, ‘If we both see the truth of your assertions and both see the truth of mine, where do we see this? Certainly not you in me, nor I in you but both precisely in the immutable truth which is above our understanding’ (Conf. 12, 25, 35). The classical text on this matter is the following: ‘…the nature of the rational soul has been made in such a way that united to intelligible things according to the natural order arranged by the Creator, it perceives them in a special incorporeal light in the same way that the bodily eye perceives that which surrounds it in ordinary light since it has been created capable of receiving this light and has been disposed towards it.’ (De Trin. 12, 15, 24)
This doctrine has been interpreted in terms of Platonic memory, of ontological intuition, of innate ideas and of the scholastic concept of abstraction. The first three interpretations do not correspond to the texts. In fact, the doctrine of illumination: a) was proposed in order to take the place of that of Platonic reminiscence (ibid.); b) excludes the immediate knowledge of God – we know God per speculum, i.e., through images (ibid., 12, 8, 14) – and thus excludes the knowledge in God of both sensible (De Gen. ad litt. 5, 16, 34) and intelligible realities (ibid., 4, 32, 49); c) supposes that the mind does not have ideas preformed in itself, but rather acquires them: ‘The human mind has thus been made that it first recognized created things as it is able, then seeks their causes, existing as immutable exemplars in the Word of God, and seeks in some way to perceive them and thus to see the invisible realities by means of created things” (ibid.).
With regard to the fourth interpretation, however, a distinction must be made. If there is intended the illuminative function of the active intellect of the scholastics conceived as a ‘participated similarity of uncreated light’ the comparison can be maintained, and it is truly a case of doctrinal continuity. Augustine insists that the human mind cannot be a light for itself (Serm. 67, 8; 183, 5). It is a light which illumines because it has itself been illuminated (In Io. 25, 3), that is, it has been created (C. Faustum 20, 7; De pec. Mer. Rem. 1, 25, 36-38). God alone is light unto Himself, and thus is the true light (In Io. 14, 1). The divine illumination establishes the certainty of our judgments and attributes to them the characteristics of universality and necessity, and thus Augustine is insistent on this illumination.”
Lydia Schumacher’s work, Divine Illumination: The History and Future of Augustine’s Theory of Knowledge, can lend a hand in explaining the theory of divine illumination,
“In this writings, Augustine suggests that the function of illumination in cognition is five-fold. Illumination serves as the source of the cognitive capacity, cognitive content, help with the process of cognition, certitude, and knowledge of God. The quotations below are organized according to these categories. Many of these passages became common citations in medieval scholastic works.
Truth is found, “in truth itself, the light of the mind.”
‘There is a mind capable of intellectual light, by which we distinguish between right and wrong.’
‘If both of us see that what you say is true and that what I say is true then where I ask do we see this? I do not see it in you, nor you in me, but both of us see it in the immutable truth which is higher than our minds…the light from the Lord our God.’
‘The things which we behold with the mind we directly perceive as present in that inner light of truth. If one sees what is true, one is being taught by the realities themselves made manifest by the enlightening action of God from within.’
‘We contemplate the inviolable truth in the light of the eternal types.’
‘The ideas are certain original and principle forms of things, i.e. reasons, fixed and unchangeable, eternal and existing always in the same state, contained in the Divine Intelligence. Though they themselves neither come into being nor pass away, nevertheless everything which can come into being and pass away is formed in accord with these ideas. It is by participation in these that whatever is exists in whatever manner it does exist. The rational soul can contemplate these ideas by a certain inner and intelligible countenance, indeed an eye of its own. In the measure that the rational soul has clung to God it is imbued in some way and illumined by Him with light, intelligible light, and discerns those reasons called ideas, or forms, or species.’
‘The earth is visible and light is visible but the earth cannot be seen unless it is brightened by light. So, likewise for those things, which everyone understands and acknowledges to be most true, one must believe they cannot be understood unless they are illumined by something else as by their own sun. Therefore just as in the sun one may remark three certain things, namely that it is, that it shines, and that it illumines, so also in that most hidden God whom you wish to know there are three things, namely, that He is, that He is known, and that He makes other things to be known.’
‘He who teaches us, namely, Christ is the Wisdom which every rational soul does indeed consult. If the soul is sometimes mistaken, this does not come about because of any defect on the part of the truth it consulted just as it is not through any defect in the light outside us that our bodily eyes are often deceived.’
‘The nature of the intellectual mind is so formed as to see those things, which according to the disposition of the Creator are subjoined to intelligible things in the natural order, in a sort of incorporeal light of its own kind, as the eye of the flesh sees the things that lie about it in this corporeal light, of which light it is made to be receptive and to which it is adapted.’
‘You have seen many true things and you distinguish them by that light which shone upon you when you saw them; raise your eyes to that light itself and fix them upon it, if you can. It is impossible, however, to fix your gaze upon this, so as to behold it clearly and distinctly.’
‘That light revealed to our interior eyes these and other things that are likewise certain.’
Knowledge of God
‘It remains for it to be converted to Him by whom it was made more and more to live by the fount of life to see light in His light and to become perfect, radiant light, and in complete happiness.’
‘The light by which the soul is illumined in order that it may see and truly understand everything is God himself. When it tries to behold the Light, it trembles in its weakness and finds itself unable to do so. When it is carried off and after being withdrawn from the senses of the body is made present to this vision in a more perfect manner, it also sees above itself that Light, in whose illumination it is enabled to see all the objects that it sees and understands in itself.’”
Schumacher continues to provide a lengthy description of the Augustinian theory of divine illumination,
“ Defining Augustinian illumination
What has been said to this point serves to bolster the contentions that illumination for Augustine is the source of an intrinsic cognitive capacity rather than any sort of intellectually offensive extrinsic conditioning. So construed, illumination evades the problems commonly associated with the claims that the divine light interferes in the process of cognition or that it imposes the very content or certitude of thoughts. By defining illumination as the source of the mind’s ability, however, I do not intend to imply that Augustinian illumination has no bearing on cognitive processes, content, or certainty. This is manifestly not the case, inasmuch as the cognitive capacity is one that must be gradually recovered as the mind cultivates a habit of reasoning in the light of faith in God.
As the mind does this, Augustine relates that it begins to employ the innate ability the Son gave to think in terms of unifying categories, in ultimate terms of the existence of one God, and thus to think in the way the Incarnate Son Himself exemplified: in the Spirit that glorifies God the Father. In the sense that the mind seeking to recover its capacity must follow Christ’s example concerning how to think, Christ affects cognitive processes, not by performing them on behalf of the mind but by putting the mind in the position to perform them of its own accord by way of the example He set at His Incarnation.
As the mind imitates Christ’s way of knowing, it gains greater insight into the object of His knowledge, which is the goodness of God the Father – not yet directly, of course, but indirectly, as it realizes the impact faith in Him has on its efforts to form ideas about reality. By forming ideas in the way the Father does, namely, through the Son and in His Spirit, the intellect increasingly participates at its own initiative in an eternal life that consists in contemplating the idea of God. While the search for God’s Truth may be in the making of the mind that undertakes it, the Truth that is discovered is not the mind’s invention. Rather, the mind through its own workings conceptually alights on an aspect of the way God has made things to be: good.
For this reason, one can affirm that illumination bears on the content of thought, not because God imposes thoughts on the human mind but because the intellect, to the extent it has recovered its capacity, comes to know what God already knows in full, which is quite simply the goodness of God, as it can be perceived through the mediation of natural experiences scrutinized from the standpoint of faith. Although the knowledge of Truth is something that is sought after ‘from below’ or through the use of the natural capacity to comprehend natural reality, one can still affirm in a qualified sense that it is something that is received from above, to the extent that the mind acknowledges that the employment of its natural capacity represents a participation in the knowledge of what is above.
The more the mind participates in the knowledge of God as it presently can, learning to see the signs of God’s goodness everywhere it turns to look, the more the mind becomes confident in the veracity of the idea it entertained from the beginning, which is that God is good. The ‘proof’ for the truth of the doctrines of the Trinity and Incarnation, consequently, is in the effects the application of those doctrines on the mind that uses them to find the good – and God – in all things, overcoming in the process the idea that the circumstances can make or break happiness while discerning how to make the best of all circumstances and find happiness in them.
As the truth of Christian faith is reinforced for the believing mind by these means, the opportunity to demonstrate its viability in the face of unbelievers also arises. For the capacity to put all things into a perspective that locates the good in them – the capacity to ‘redeem’ them – is a testament to the powerful effect that faith in the Triune, Incarnate God can have whenever it is invoked. While those who are aware that God is an all-inclusive good can identify the sense in which God can bring good from virtually anything, and in that, find a way to overcome difficulties and reconcile differences in perspective, those that are not aware of the all-inclusive nature of that light, who tend to reduce it to some finite light, do not have the resources to embrace all that surrounds. By making use of the resources of faith to redeem the circumstances and incorporate the ideas of others, as Augustine did with the ideas of the Platonists, for example, the people of faith acquire a charitable attitude of open-mindedness that is conducive to promoting unity and peace and that serves as the source of their faith’s pervasive power.
That attitude is one of the effects of faith in God, which provides perhaps the most convincing evidence for the truth of Christian doctrine that can be produced in an order where God Himself is never fully disclosed. Since those effects can only be identified by a human mind that is affected by faith and that is prepared to give an account of the object of faith by which it is affected – the Triune God – and how it is affected – through the Incarnation of God’s Son – Augustine insists that those wishing to lead others to belief in God must go about this in the way Christ modeled: not by shining the light of faith on the eyes of those who reason in the dark, but by showing how effective it is to walk in the light of what makes the way forward clear and fosters fellowship with others. He urges his readers to persuade others to believe through the application of the belief in the goodness of God, which produces certainty about the goodness of all that happens in reality, which in turn reinforces belief in the goodness of God. Here, illumination can be said to afford cognitive certitude not because this is imposed from the outside but because the mind that recovers its capacity inevitably experiences a directly proportional increase in certainty with respect to belief in God. The certainty that results from seeing reality by the light of faith doubles as the confidence in the Light Itself that remains as yet unseen but will surely be seen by the eyes that adjust to it by faith.
All this may be summarized by saying that divine illumination is the source of an intrinsic intellectual capacity all human beings have to illumine the nature of God. So construed the theory evades the problems typically associated with interpretations that treat the divine light as though it were some sort of intrinsic force. Those interpretations have not done justice to the later developed theological context of the account Augustine most famously mentions in early ‘philosophical’ works. Inasmuch as the capacity that comes through illumination is one that must be gradually recovered, however, it is possible to affirm that illumination enters into cognition in the three other ways Augustine admittedly mentions, namely, as an ongoing help in the cognitive process and as the source of cognitive content and of certitude. This is not because Christ the illuminator directly instigates or interferes with the cognitive process or imposes ideas and certainty about them, but because the human mind can only recover its capacity by following the example He set through engagement in a process of cognition that is analogous to His and that results in a growing understanding of and certainty about the Being of God that He always knows in full.
With all this in view, one can conclude that the illumination of Christ does not bear on cognition in any way that undermines the autonomy or integrity of the intellect but in a way that reinstates it, at least for the intellect that stokes rather than extinguishes His light through a decision to work with faith in Him. On Augustine’s account, all that comes to the intellect from the outside is the power to be renewed on the inside; this is the power to illumine the divine being that is received through divine illumination – the power to know like God and thus know God. Here at last is the logic of Augustine’s claim that divine illumination is the condition of possibility of all human knowledge comes into relief – for unless God gives the capacity to know Him and it is used to the end of knowing Him, there is no such thing as knowing or knowledge at all. After all, there is nothing to see in the dark.”
While these authors provide a good starting point for analyzing Augustine’s theory of divine illumination, there is still much left to investigate. In addition to coming to grips with Augustine’s theory, an exploration into how his thought shapes the theological and philosophical projects of important figures such as Anselm, Aquinas, and Bonaventure is also important.
– Lucas G. Westman
 “In order to reconstruct the fundamental lines of Augustine’s philosophy it is useful to keep in mind the principles which inspired and qualified it. In the judgment of the present author these principles are substantially the following three: interiority, participation, immutability.” Patrology Volume IV, Quasten, Pg. 407
 Ibid, Pg. 408
 Ibid, Pg. 420, 421
 This text is useful in understanding Augustine’s theory, however, it follows an unnecessary trend in academia attempting to link Augustinian theological traditionalism to Aquinas at the expense of the Franciscans, especially Bonaventure and Scotus. While it is the case that Scotus moved away from illuminationism, making Bonaventure and Aquinas out to be theological competitors on the veracity of their Augustinian approach to their thought creates an illegitimate divide between these two great Saints and Doctors of the Church.
 Schumacher, Pg. 4 – 7
 Ibid, Pg. 62 – 65