Critical realism is not always easy to explain. It is, most basically, an insight into the fundamental nature of things. Specifically, it is an insight into the fundamental nature of this thing that is human knowing. It is the fruit of what Lonergan describes as intellectual conversion. In intellectual conversion, the human subject apprehends--not just theoretically, but deep in the bones--that knowing is not like looking. It entails rejecting the empiricist's false conviction that we only what we can look at is real and thus can be known, and the idealist's equally false conviction that the work of looking obviates the possibility of knowing. In historical Jesus studies, my primary area of expertise, these false convictions respectively play themselves out on the one hand in the search for criteria that will exclude from our field of vision that which is not real, and on the other in the recurrent despair of ever being able to apprehend the Jesus of history. Critical realism moves beyond this by affirming rightly that while knowing is not like looking, looking (and hearing, smelling, tasting, touching: experiencing in general) is a foundational and indispensable step in knowing. We infer reality from that which we experience. For those who have experienced this intellectual conversion, critical realism will present as self-evident; for those who have not, it will present as really quite baffling.
Through the work of empirical (not empiricist) inquiry, we come to know that which cannot be experienced by attending to that which can. We call that which can be experienced "data," and that which we can know from experience we call "reality." I was recently reading in Romans, and was struck by the way in which Paul articulated this understanding of knowledge remarkably well, the better part of two millennia ago. In Romans 1:20a, Paul writes that "Ever since the creation of the world [God's] eternal power and divine nature, invisible though they are, have been understood and seen through the things he has made." The data: the things that God has made. The inferred reality: God's eternal power and divine nature. Paul's assertion is that while we cannot experience God's eternal power and divine nature directly, we can apprehend it through our experience of God's creation. The crucial step for the critical realist is recognizing that it is this act of apprehension that constitutes genuine knowing. The act of experiencing creation, that is a necessary step on the path to this apprehension, but the apprehension of the eternal power and divine nature is the real act of knowing.
It would take more than the above, quick analysis to suggest that Paul was a critical realist avant la lettre. Even if he were, he would have been one "at the level of this time," i.e. one who operated two millennia ago, without the benefit of the many insights that we have stumbled upon since in our collective project of better apprehending our shared reality. In any case, Paul's consciousness of his own subjectivity and the work of knowing (as interesting as it might be in its own regard) is not really my interest here. My interest is simply in using what Paul wrote to help elucidate the basic insight that drives critical realism. Indeed, Paul could in principle be mistaken in his conclusion about God's power and nature, without obviating the basic understanding of knowing that he evinces. What matters for my purpose here is the work of inferring reality from what is experienced, without falling either into the error of confusing the experience with reality or the alternative error of severing the two completely.