Hume is correct that moral obligation cannot be derived from purely theoretical reason—from comparing ideas and perceiving their relations. But he is wrong to conclude that morality is therefore grounded in sentiment. Practical reason, distinct from theoretical reason, legislates the moral law through the categorical imperative. This is neither a matter of comparing ideas nor of consulting feelings, but of recognizing what can be willed as universal law. Moral obligation is rational, but it is practical rationality, not theoretical. The moral law is synthetic a priori—it applies necessarily to all rational beings, but it is not derived from the analysis of concepts. Hume's failure to distinguish practical from theoretical reason leads him to moral sentimentalism.

This is moral skepticism of the most dangerous kind. Hume denies that reason can discern moral truths, reducing morality to sentiment and feeling. But we do perceive moral truths through reason—that justice is obligatory, that gratitude is due to benefactors, that we should keep promises. These are not matters of sentiment but of rational insight into the nature of things and the fitnesses between actions and circumstances. A benevolent action toward someone who has shown us kindness is fit and appropriate; an act of cruelty toward an innocent person is unfit and wrong. These moral truths are perceived by reason just as mathematical truths are. Hume's sentimentalism destroys the objectivity of morality and makes moral disagreement unintelligible.

Hume's denial that moral distinctions are derived from reason leads him to ground morality in sentiment—in how actions make us feel. This subjectivizes morality and removes its divine foundation. Moral truths are indeed rational truths, known through the understanding. They reflect the will of God, the eternal moral law that governs creation. We perceive moral truths through reason, though not through purely abstract reasoning. We must understand the nature of God, the purpose of human life, and the relationships between actions and human flourishing. Hume's sentimentalism is another manifestation of his skepticism, denying the rational foundations of morality just as he denied the rational foundations of causation and substance.

Hume exaggerates when he denies any role for reason in morality. Moral rules can be demonstrated through reason when we understand the nature of God, human nature, and the relations between actions and happiness. The law of nature is discoverable by reason—it prohibits actions that harm others and commands actions that promote the common good. However, I agree with Hume that morality must connect with motivation and action. If moral rules are to govern conduct, they must be enforced by sanctions—rewards and punishments in this life and the next. Divine law provides these sanctions, giving us self-interested reasons to be moral. Reason discovers moral truth, but motivation comes from concern for our own good, which reason shows is connected with moral behavior.

Hume here identifies a genuine problem—you cannot derive an 'ought' from an 'is' through mere logic. But his conclusion should be that morality has its own rational foundation independent of theoretical reason's descriptions of facts. The categorical imperative is not derived from facts about human nature or the world; it is a principle of pure practical reason. We do not infer what we ought to do from what is the case, but from what reason demands. The moral law commands categorically, independently of facts about desires, consequences, or natural properties. Hume rightly sees that morality cannot be naturalized, but wrongly concludes that it must therefore be grounded in sentiment rather than in practical reason.

Hume is correct that we cannot derive moral obligation from mere descriptive facts through logical deduction. But this does not mean that moral truths are not rationally knowable. Moral truths are first principles, known through rational intuition rather than through inference from non-moral facts. When we perceive that gratitude is due to benefactors, we are not inferring this from facts about human psychology or social utility; we are directly apprehending a moral truth. These moral first principles are self-evident to rational beings, just as mathematical axioms are. The attempt to derive 'ought' from 'is' fails because moral principles are foundational, not derivative. But this makes them no less rational or objective.

Hume identifies the gap between fact and value, between 'is' and 'ought,' but draws the wrong conclusion. The 'ought' cannot be derived from natural facts precisely because moral obligation has its source in God's will. Moral laws are not natural laws that describe how things do behave, but divine laws that prescribe how rational creatures ought to behave. These laws are known through reason, not through observation of nature. We perceive through rational insight that we ought to obey God, promote the good, and avoid evil. The 'ought' is rationally apprehended, even though it cannot be deduced from descriptive premises. Hume's observation should support theological ethics, but he uses it instead to argue for moral sentimentalism, missing the divine foundation of moral obligation.

The transition from 'is' to 'ought' is indeed crucial, and Hume is right to demand explanation. The bridge is provided by understanding God's will and purpose. Once we establish that God exists and that He created humans for certain purposes, we can derive what we ought to do—namely, what conforms to God's law and promotes the purposes for which we were created. The 'ought' comes from divine command and from the natural law that reflects God's reason. Without this theological foundation, I agree that we cannot derive obligation from mere facts about human nature or the world. Hume's skepticism about the is-ought transition should lead us to recognize the necessity of grounding morality in divine law, not to embracing moral sentimentalism.