[Essay II xxii 10...

[Essay II xxii 10, 12] But notice that these ideas, and the words that signify them, can be fully formed in the mind independently of their actual instantiation. We can know what sacrilege and resurrection would be without experiencing their occurrence; we choose to define single words for "murder" and "stabbing" but not for other ideas equally familiar in experience; but we form abstract ideas of those human actions to which we most commonly refer, whether or not they frequently occur.

[Essay III v 5-7] Thus, on Locke's view, such moral ideas as those of obligation, drunkenness, or lying, are formed by combining simple ideas from the mental and physical aspects of human nature without ever supposing that anything conforming to the new composite has ever existed. [Essay II xxii 1] This detachment from questions of real existence, Locke believed, is crucial for establishing the demonstrable status of human morality.

Demonstrable Rules The moral rectitude of actions of a particular sort, Locke held, is wholly constituted by the demonstrable relation between our clear ideas of such actions and the equally clear conception of the moral law. Indeed, this relation is often so obvious-as, for example, in the cases of "murder" and "theft"-that the moral condemnation comes easily to be included as a part of our complex idea of the action itself.

[Essay II xxviii 14-16] Because both my contemplated action and the moral rule can be abstractly conceived as mixed modes, the applicability of this rule to that action can be determined with perfect certainty. It is a further question whether or not the moral rule itself is demonstrably true. Locke believed that it often is. To be sure, reliance upon an axiomatic deduction of morality from a fixed set of putatively indubitable first principles would be neither effective nor intellectually sound.

[Essay IV xii 4-5] Nevertheless, demonstration is possible in principle wherever we have clear ideas, and Locke was careful to emphasize that indubitable knowledge of relations does not presuppose perfect clarity with respect to the relata. We might know that one automobile is faster than another, for example, even if we had little information about the mechanical differences that produce this result.

Our awareness of relations commonly rises to a level of certainty greater than our knowledge of the things among which they hold.