Augustine's concept of the deep self provides a basis for a complex and many-faceted account of critical thinking. He uncovers the moral sources of thinking in the inner depths of the self and shows that critical thinking presupposes radical self-reflection ready to face the truth about oneself. Self-knowledge assumes transparency, consciousness of the corrupt desires and prejudices that distort one's thinking. Unresolved guilt endangers transparency and thereby makes it difficult to become aware of the vices distorting one's perspective on reality. That is why human beings need divine grace that gives them the courage to face their corruption. For Augustine, the problem of critical thinking is part of a larger problem about how the human self and identity are formed, which factors influence the process, and how a person comes to know herself. Augustine writes an open account of his life in order to clarify this problem. His intention is to make sense of the nature of his self by thinking carefully who he is and how he became who he is. Augustine seeks to find an answer to this question both philosophically and autobiographically, by analysing the factors that influenced the formation of his own identity and the development of his self-knowledge and by reflecting philosophically on the nature of these influences. Reason is one essential part of the human soul. Since God has given reason to human beings, it must have a purpose. Augustine seeks to clarify this purpose by reflecting on fundamental epistemological questions: What is knowledge and where does it come from? What is the relationship of human reason to knowledge? How can one reach ultimate knowledge? According to Augustine, human reason and perception have been formed to acquire knowledge about reality. If God had not made human reason and perception fitting for their task, knowledge would be completely unattainable. Since God has made human reason capable of acquiring reliable knowledge, reason has an important task in the spiritual development of human beings. It is especially useful when trying to make clear conceptual distinctions. Reason does not, however, function independently of the will and the emotions. For reason to acquire a reliable grasp on reality and to understand things properly, the human heart must love the truth, the good and the right sufficiently to face its own prejudices and to gain self-knowledge. Critical thinking has, therefore, certain crucial preconditions, according to Augustine. The aim of this article is to clarify the structure of these preconditions. (1) In order to think critically, one has to distinguish between how reality appears to one and how it is in fact. (2) There is a close connection between willing and thinking, between one's deepest desires and one's view on reality. (3) One cannot distinguish reality from appearances unless one realizes how corrupt desires and prejudices distort one's perspective on reality. (4) In order to be able to face one's evil desires and become conscious of their distorting influence, one needs the courage to face one's depravity. Such a courage presupposes God's grace and his promise of forgiveness, since without divine grace human beings try to cover up the truth about themselves and remain unconscious of the distorting influence of their evil desires. (5) One needs a source of light that enlightens the deep recesses of the self and shows it in the true light but is yet external to the human being and independent of him. (6) This source of inner light has to be of a personal nature to provide the learner with the possibility of inner dialogue. Augustine assumes that God is the inner teacher of every human being. A crucial factor in the development of critical thinking is that one becomes more dialogically engaged with the inner teacher.