Abstract: While much scholarly attention has been given to John Locke's political theory found in his Two Treatises of Government (1689), substantially less attention has been given to his moral philosophy hinted at in An Essay Concerning Human Understanding (1689). In the Essay, Locke argues that "if duly considered and pursued…[the] foundations of our duty and rules of action… might place morality amongst the sciences capable of demonstration." In his own day, William Molyneux and Catharine Trotter Cockburn called upon Locke to explain this claim by writing a treatise about morals. Since Locke died before he could write such a treatise, what Locke means by this claim is still a topic of debate among Locke's contemporary interpreters. Therefore, how Locke intended to make morals demonstrable is still an open question.

The purpose of this paper is to use Locke's statements about morals in the Essay to explain what he means when he claims that morality is capable of demonstration. To do this, we must understand what he means by morality and how his concept of morality applies to his theory of demonstration. As such, I will argue that, for Locke, the morality of an action is the feature of that action that makes it praiseworthy or blameworthy when it is found to agree or disagree with a general rule. Additionally, our ideas are considered demonstrated when there is sufficient evidence to show a true relationship between some of our ideas. Therefore, to make a moral demonstration, we must be able to provide a proof (i.e., a series of well-reasoned steps) that establishes the truth of a moral relation.

In this paper, I will argue that when Locke says that morals are capable of demonstration, he means that the rightness or wrongness of an action must be established by two demonstrations. In the first demonstration, we establish that our idea of an action is either right or wrong in so far as it agrees or disagrees with the content of a general rule. Then, in the second demonstration, we establish the validity of that general rule by demonstrating agreement or disagreement between our ideas about the consequences of obeying that rule and our ideas about what will produce pleasure or pain.

In section 1, I will explain how Locke defines demonstration as a proven agreement among our ideas, and I will explain what it means for morality to be capable of demonstration. I will then explain how Locke makes the first kind of demonstration to prove that our idea of an action agrees or disagrees with the content of a general rule. Then, in section 2, I will argue that Locke believes that the true rules of morality are divine laws that are promulgated to us as laws of nature that can be discovered using reason. I argue that to discover these valid rules of morality Locke intends us to use reason to make a second demonstration. This second demonstration aims at proving the relation between our ideas about the consequences of following those general rules and our idea of goodness as that which produces pleasure. I conclude from this, that by following this procedure, Locke is able to both demonstrate the truth of a moral proposition by showing that morals can be rationally demonstrated and well-grounded as a law of nature.