“Reason” was the ultimate court of appeal for deists. Tindal presents a Lockean definition of reason, self-evident truth, and the light of nature:
By the rational faculties, then, we mean the natural ability a man has to apprehend, judge, and infer: The immediate objects of which faculties are not the things themselves, but the ideas the mind conceives of them…. Knowledge [is]… the perception of the agreement or disagreement of our ideas. And any two of these, when joined together so as to be affirmed or denied of each other, make what we call a proposition… Knowledge accrues either immediately on the bare intuition of these two ideas or terms so joined, and is therefore styled intuitive knowledge or self-evident truth, or by the intervention of some other idea or ideas …… this is calleddemonstrative knowledge…
If there were not some propositions which need not to be proved, it would be in vain for men to argue with one another [because there would be no basis for demonstrative reasoning] … Those propositions which need no proof, we call self-evident; because by comparing the ideas signified by the terms of such propositions, we immediately discern their agreement, or disagreement: This is, as I said before, what we call intuitive knowledge…. [Intuitive knowledge] may, I think, be called divine inspiration as being immediately from God, and not acquired by any human deduction or drawing of consequences: This, certainly, is that divine, that uniform light, which shines in the minds of all men…
—Matthew Tindal, Christianity as Old as the Creation (II)
Deists did appeal to “the light of nature” to support the self-evident nature of their positive religious claims.
By natural religion, I understand the belief of the existence of a God, and the sense and practice of those duties which result from the knowledge we, by our reason, have of him and his perfections; and of ourselves, and our own imperfections, and of the relationship we stand in to him, and to our fellow-creatures; so that the religion of nature takes in everything that is founded on the reason and nature of things. I suppose you will allow that it is evident by the light of nature that there is a God, or in other words, a being absolutely perfect, and infinitely happy in himself, who is the source of all other beings….
—Matthew Tindal , Christianity as Old as the Creation (II)
Once a proposition is asserted to be a self-evident truth, there is not much more to say about it. Consequently, deist authors attempted to use reason as a critical tool for exposing and rejecting what they saw as nonsense. Here are two typical examples. The first is from John Toland’s Christianity Not Mysterious.
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