The Cartesian Method
Rene Descartes (1596-1650), often recognized as the first of the modern philosophers, proposed a particular epistemological framework within Meditations on First Philosophy (1641) and Principles of Philosophy (1644). These principles are what have established the basis for intellectual inquiry in the following centuries. Descartes’ most significant philosophical contribution pertained to doubt which he employed as a method to obtain truth. He wished to discover a foundational basis which could withstand critical scrutiny, and on which he could also construct knowledge. Descartes discounted whatever he possibly could, and following his method consistently, he found he could doubt that of which most people consider reasonable. This included one’s own sensory experience, the existence of the external world, and the objects within it. There was an exception to this, however, as Descartes reasoned that one could not reasonably doubt one’s own existence for if one did then who would be doing the doubting? This he expressed in the cogito ergo sum (“I think, therefore I am”), and so Descartes constructed a foundation on which to build knowledge of the self, the world, and God. This Cartesian Method would greatly influence Immanuel Kant (1724-1804) because it demonstrated a method through which one could reduce complex entities into simple components. Although Descartes did so in terms of epistemology it could too be done with an analysis of religion. As Kant would see, one could attempt to whittle down religion’s elaborate structure to a foundational base. Thus, Descartes provided a conceptual system for the study of religion.
The Kantian Paradigm
Immanuel Kant, active during the period of the Enlightenment, was the primary practitioner of Descartes’ method. Kant was active within a period within which scholars attempted to differentiate between natural religion and revealed religion (more on how this applies to Kant in a second) (Capps 1995, 7). Typically understood, natural religion was that religion which was common to all human beings simply by virtue of their humanity. This religion was accessible, inviting, tolerant, available to all, and possessed no ecclesiastical authorities. To the contrary, revealed religion was that religion intrinsically linked with institutions such as churches, and was accompanied by specific authorities, doctrines, creeds, theologies, and liturgies. Many Enlightenment thinkers saw the latter as tyranny and intellectual enslavement.
Influenced by Descartes, Kant developed a method to identify the core of religion, or religion’s core, fundamental element (Capps 1995, 7). Religion was to be analyzed in a reductionist manner if it was to made intelligible and situated within fundamental human capacities. Kant attempted to reach certifiable first principles of religion beyond the excesses of revealed religion and reasoned that the essence of religion was possibly committed to one of three (tripartite) fundamental human capacities, namely, the rational, ethical, and the beautiful (Capps 1995, 12). Kant appeared most interested in the ethical. He thus emphasized the ethical as the primary, essential category of religion, and therefore viewed religion as being a “matter of feeling.” He argued that the human capacity to be moral would only be partially developed unless it was somehow influenced by religion (Capps 1995, 11). If not then the human moral capacity would suffer from impoverishment and/or misalignment. Kant worked extended beyond first principles through his employment a two-part approach: reductio and enumeratio (Capps 1995, 12). In Religion Within the Limits of Reason Alone (1793), he closely married religion with ethical and moral sensitivities, and transitioned from this primary category (reductio) to enumeratio. Through enumeratio, Kant explained the teachings of the Christian religion. He too articulated and defended some of the central doctrines of Christianity which led him to argue that the Christian religion was philosophically compelling and morally self-consistent.
The Kantian Paradigm left scholars after Kant with three possibilities, namely, to  develop Kant’s own position further (strengthening and/or expanding it), to  seek after alternative possibilities within the tripartite capacities (some scholars suggested that rationality and/or aesthetics to be a more appropriate category), or to  seek an altogether different paradigm. The final possibility led scholars over subsequent centuries to propose alternative paradigms for understanding religion (Chidester 1983, 55-56). Either way, Kant’s paradigm became the major framework through which to study religion.
Rudolf Otto’s Category of the Holy
A prominent theorist to look for a first principle (sine qua non) of religion was German theologian and philosopher Rudolf Otto (1869-1937) who saw religion as supremely transcendental (Chidester 2011, 85). Otto suggested that there were significant limitations when attempting to understand deity through ratiocinative method, and argued that deity could not be sufficiently understood through rationality (Capps 1995, 20). Otto claimed that irrational elements belonged to the heart of religion, and that by applying too much rationality one would produce an inaccurate portrayal. He saw the Christian religion and its doctrines as “one-sidedly intellectualistic and rationalistic”, and therefore as an inaccurate portrayal of religion. Otto thus proposed an additional fourth a priori category, namely, “the Holy.” He argued that “holiness” was unique to religion, most closely associated with goodness, and that it should be distinguished from rationality (Capps 1995, 21). Without the Holy, which he too referred to as the “numinous,” religion would not exist (Capps 1995, 22). The numinous, he proposed, was an intangible and unseen yet compelling reality that inspired both fascination and dread within human beings, and that was always present within religious experience and awareness. It also encompassed the irrational and non-rational core of religion that pointed to a reality outside of oneself. In order to flesh out his category, Otto posited the numinous to consist of two elements which were bound together, the tremendum mysterium (Capps 1995, 22-23). By tremendum he meant awe, majesty, and urgency. By mysterium he meant something wholly other and distinct from everything else but despite such distinctiveness, the mysterium still attracts and fascinates. According to his evolutionary concept of religious consciousness, Otto believed that the first human beings acknowledged the numinous, but only the fearful side of it (as represented in their fear of divine wrath) (Capps 1995, 23). They soon, however, became aware of another side of the numinous, namely, “positive self-surrender to the numen.” Otto suggested that Christianity was superior in respect to all other “sister” religions because it bound together harmoniously the non-rational and rational elements of religion.
Capps, Walter. 1995. Religious Studies: The Making of a Discipline. Minneapolis: Fortress Press,
Chidester, David. 1983. “Aesthetic Strategies in Western Religious Thought.” Journal of the American Academy of Religion 51(1): 55-66. Accessed February 15, 2019. https://www.jstor.org/stable/1462898
Chidester, David. 2011. “Sacred.” The Journal of Objects, Art and Belief 7(1): 84-90. Accessed February 15, 2019. https://doi.org/10.2752/175183411X12968355482132: