Philosophical theology is both a branch and form of theology in which philosophical methods are used in developing or analyzing theological concepts. It therefore includes natural theology as well as philosophical treatments of orthodox and heterodox theology. Philosophical theology is also closely related to the philosophy of religion. Mortimer J. Adler distinguishes philosophical theology from natural theology. While the former is, according to him, theology done by non-Christian philosophers, the latter is done by those who are already Christians and are actually trying to find rational proofs for their faith. Adler thinks that the term "natural theology" is a misnomer and is actually apologetics, and cites as an example Thomas Aquinas' Summa Contra Gentiles, addressed to the Jews and Moors in Spain, which though written as if it were philosophical theology, was in fact, apologetics and not philosophical theology, as it was written by a Christian and not by a pagan. In Adler's view, few if any of the books on natural theology, are sound as works in philosophical theology, because they are not written by pagans. An example of philosophical theology, according to Adler, is Aristotle's theology as found in his Metaphysics. Contrary to Adler's view, Charles Taliaferro and Chad Meister think that doing philosophical theology may not be restricted by whether the doer is an insider to faith or not, for anyone doing philosophical theology only tries to consider whether a theological doctrine can make philosophical sense or not. But, as Adler points it out, a pagan doing philosophical theology and coming to a theological conclusion has a different route than an insider using philosophy to examine or seek to understand his faith. The relationship between theology and philosophy has been long-debated and discussed with the Christian tradition. Tertullian, an influential early Christian theologian and apologist, believed that philosophy has little to do with theology, arguing that the use of philosophy often corrupted theology, leading to unorthodox beliefs that were not grounded in the early Christian tradition.