A person can check a proof in geometry or accept a statement because they trust the source. John Locke uses that difference to explain faith and reason in An Essay Concerning Human Understanding. His examples include a theorem about triangles and the Christian expectation that the dead will rise. They help distinguish checking a claim directly from deciding whether to trust a claim passed down as revelation. John Locke, An Essay Concerning Human Understanding, book IV, chapter 18
What Locke means by reason and faith
In book IV, chapter 18, Locke describes reason as a way of discovering the certainty or probability of a proposition using ideas and faculties available to us. In the sense he is using in that chapter, faith means accepting a proposition on the authority of God as its source. He calls the communication of the proposition revelation. That usage narrows the discussion to a particular reason for believing something. Everyday uses of faith can also refer to trust in people, practices or commitments. John Locke, An Essay Concerning Human Understanding, book IV, chapter 18
What a triangle and a resurrection claim show
Locke imagines being told that it was revealed long ago that the angles of a triangle sum to two right angles. Within Euclidean geometry, a reader can work through a demonstration of that result. For Locke, seeing how the result follows gives a kind of certainty that a report about its revelation cannot supply. The report adds another question, whether the account was transmitted accurately and actually came from God. John Locke, An Essay Concerning Human Understanding, book IV, chapter 18
Locke takes the resurrection of the dead as an example of a claim above reason. He means that a person cannot establish the event through the ordinary use of their faculties. In his account, a claim of that kind can be accepted by faith if it is divinely revealed. When a claim is contrary to clear knowledge, he gives priority to the knowledge. The difference between above reason and contrary to reason matters because a lack of a proof is different from a demonstrated contradiction. John Locke, An Essay Concerning Human Understanding, book IV, chapter 18
Why a reported revelation still calls for judgement
Locke distinguishes a communication made directly to a person from one passed on to others in speech or writing. A later reader has to judge whether the statement was revealed and what the transmitted words mean. Locke assigns that judgement to reason. In his account, genuine revelation can settle an issue on which human reasoning yields only a probable conclusion. The reader still has to assess whether a given report qualifies as revelation. John Locke, An Essay Concerning Human Understanding, book IV, chapter 18
Conclusion
Locke separates the evidence for a proposition from the evidence for trusting its reported source. A geometric proof can be checked directly. A statement received as revelation calls for a judgement about its origin and meaning, even when the claim itself is beyond what reason can demonstrate. That distinction explains how Locke gives reason a role in the act of faith. See also our broader article on conflicts between faith and reason.
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