What are the relationships between different departments of knowledge? I suppose that one problem with this question is the human habit of compartmentalising what we do. I am a physicist; that means that I know (or should know, professionally) little or nothing about history, theology, philosophy, or geography (to name but a few). I might, but my knowledge is that of an amateur.
This problem is sometimes painfully obvious. For example, the highest-profile atheist of recent times, Richard Dawkins, is a professional developmental biologist. However, he developed a tendency to write outside that area, discussing philosophical ideas and a-theology. It became evident to experts in these fields that he did not really know what he was talking about or dismissing. I believe he is on record as claiming that theology is not an academic subject, which seems to me to be simply an admission that he has not read any.
The possibilities of shooting oneself in the foot if we stray outside our expertise are many and varied. A number of theologians I have read comment on science, for example. What becomes clear is that, mostly, they have not done any science and, therefore, make mistakes that a scientist in the area would not make. Similarly, scientists straying into theology can make howlers which their new colleagues probably shake their head over. That does not mean that there is no insight into these areas by those from outside, it is just that it is very easy to make mistakes.
If there were enough time in a life, we would want to study every area before blundering in with our own views and arguments. Sadly, this is not open to us. At most, we can be expert in one field and a well-read amateur in another. This does not absolve us of the responsibility of trying, of course, but it does make us rather humble in doing so.
And yet there is the nagging feeling that these fields can and do link up. A number of examples are obvious: chemical physics is a subject incorporating some aspects of both subjects. Health geography (if I really knew what it was) would probably be another area. These neatly categorised areas of thought are found to link up and, sometimes, that linkage can be highly productive. But, even so, the links are not strong and the boundaries between one subject and another are sometimes jealously guarded.
F. R. Tennant, in his Philosophy of the Sciences, has a decent stab at a modern hierarchy of the sciences. This book started as the Tarner series of lectures in 1931-2. Tennant suggests that there is an order of knowledge and it that starts with psychology. Psychology, he argues, is primary because it is the first thing to do with thinking, thought, and knowledge. It is interesting to note as well, that Tennant seems to consider every subject a science. Whether this is a generous spirit or deference to the advance of the physical sciences I am not sure.
Tennant goes through various subjects. For example, he discusses the debates around whether history is a science or not. That, of course, turns on what you think science is in the first place. Physical scientists would probably be inclined to say no, and indeed, scientific method of hypothesis, experiment, and verification probably is not that easy to apply to history. After all, we cannot re-run the battle of Waterloo and see if the outcome is different. On the other hand, critical analysis of documents, formulation of hypotheses to fit the facts, and determination of the fit would be recognisable to history. This is not a million miles (light years?) away from astrophysics which starts with observation and develops hypotheses from them and physical theory and then checks back with the observations.
Tennant goes on the discuss the relations of natural science with logic and mathematics (which he calls ‘pure’ sciences). Incidentally, I am not convinced that these pure sciences are so pure. Mathematics often takes its cue from ‘how do I do this?’ where the ‘this’ is, for example, build a pyramid. Tennant sees logic and mathematics as instrumental to the natural sciences. Nor do the pure sciences inform philosophy or metaphysics. Tennant concludes that there is no hierarchy of the sciences and that any order is a matter of the interest humanity puts on a specific area of knowledge.
Theology, as is well know, used to be the ‘Queen of Science’ and was dethroned during the Enlightenment, if not sooner. However, theology is not isolated from other subjects. Views that it is lead into difficulties. Religious experience, for example, does not offer the establishment of the uniqueness or reality of the experience. A religious experience is, necessarily, subjective and the question from an external observer can always be ‘to what object does this experience refer?’ If the experience is of God then, as God is not an object in the universe (despite the claims of some a-theologians) the experience cannot be of anything objective. The experience is a real one, but the content of it cannot be established, at least by our normal methods of establishing the reality of experience.
Tennant argues that theology is a generalisation of ordinary knowledge or world and man, synthesised into a reasonable belief in theism. Science itself, he argues, is a venture in faith, faith in the reliability of the world. Theology, on his view, is a self-completion of science, a creation of a philosophical world view. I would grant that science is a venture in faith, but not that science can be completed in any sense. Every time humanity has thought that science is nearing completion another world of science opens and its implications seem limitless. On that basis, it seems to me that theology cannot complete anything, but is similarly open to new insights, whether from science or other areas of human experience. One of the lessons of theology in recent decades is that the deliverances of science are to be taken seriously, but not absolutely. Science delivers new models and concepts, not absolute unshakable truth.