revelation to us but can also be known through reason. And Aquinas proceeds to show
how reason can establish them. What we would today call philosophy of religion (or
natural theology) is thus an integral part of his systematic theology. Early modern
philosophers like Descartes, Leibniz, and Locke are only incidentally concerned with
purely theological issues, but they too insist that some important truths about God can be
established by purely philosophical reflection.
The notion that we should accept only those religious beliefs that can be established by
reason was not commonly expressed until the later part of the seventeenth century,
however, and not widely embraced until adopted by the eighteenth-century
Enlightenment. The consequences of the new commitment to reason alone depended on
whether important religious truths could be established by natural reason. Deists believed
that they could. Human reason can prove the existence of God and immortality and
discover basic moral principles. Because these religious beliefs are the only ones that can
be established by unaided human reason, they alone are required of everyone. They are
also the only beliefs needed for religious worship and practice. Beliefs wholly or partly
based on some alleged revelation, on the other hand, are needless at best and pernicious
at worst. Others, such as Hume, adopted a more skeptical attitude toward reason's
possibilities. In their view, reason is unable to show that “God exists” or that any other
important religious claim is significantly more probable than not. The only proper
attitude for a reasonable person to take, therefore, is disbelief (atheism) or unbelief
(agnosticism). The result of this insistence on reason alone was thus that religion either
became desiccated, reduced to a few simple beliefs distilled from the rich traditional
systems that had given life to them, or ceased to be a live option.
Reaction was inevitable, and took two forms. One was a shift from theoretical to practical
(moral) reason. Kant, for example, was convinced that “theoretical” or “speculative”
reason could neither prove nor disprove God's existence or the immortality of the soul.
Practical reason, on the other hand, provided a firm basis for a religion lying within the
“boundaries of reason alone.” The existence of God and an afterlife can't be established
by theoretical reason. A belief in them, however, is a necessary presupposition of
morality. Others, such as Friedrich Schleiermacher, shifted their attention from
intellectual belief and moral conduct to religious feelings and experience. In their view,
the latter, and not the former, are the root of humanity's religious life. Both approaches
were widely influential in the nineteenth and early twentieth century. The first fell into
neglect with the waning of philosophical idealism in the first half of the twentieth
century, although interest in it has recently resurfaced (see chapter 14). The second has
continued to be attractive to many important philosophers of religion (see chapters 6 and
10).
Philosophy of religion was comparatively neglected by academic philosophers in the first
half of the twentieth century. There were several reasons for this. One was the
widespread conviction that the traditional “proofs” were bankrupt. Be
end p.
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