In his article, “Reason and Ethics in Hobbes’s Leviathan,” John Deigh makes two principal claims.  The first is that what he calls the orthodox interpretation of Hobbes’s ethics must be rejected because it is inconsistent with Hobbes’s account of reason.  The second is that what he calls the “definitivist” interpretation of Hobbes’s ethics should be accepted because it renders the various components of Hobbes’s position consistent.  In his response to Deigh’s article, Mark Murphy in effect attacks both claims.  He attacks the first by arguing that the orthodox interpretation of Hobbes’s ethics is indeed consistent with Hobbes’s account of reason.  And he attacks the second by arguing that the definitivist interpretation of Hobbes’s ethics in fact fails to equip Hobbes with a consistent view because it is inconsistent, in particular, with what Murphy calls the “essential normativity” that Hobbes attributes to the laws of nature.
        I will focus my remarks here entirely on Murphy’s response to Deigh’s second claim.  There are three reasons for this.  The first is that I believe that Murphy is correct to reject Deigh’s first claim, and would have little to say about it other than that he is correct.  The second is that it is the second of Deigh’s two claims that strikes me as the more important.  Even if Deigh is mistaken in thinking that the orthodox interpretation renders Hobbes’s system of thought internally inconsistent, if he is nonetheless correct in thinking that the definitivist interpretation also renders Hobbes’s position consistent, then he may still prove to be warranted in believing that that definitivist interpretation is superior.  The third is that although I agree with Murphy that the definitivist interpretation should be rejected, I am not completely satisfied that Murphy himself has justified this assessment.  So I will briefly recount what I take to be Murphy’s central argument against Deigh’s second claim and then suggest a response to it that a defender of the definitivist account may be in a position to offer.
        Murphy’s argument rests on two claims.  The first is that, on Hobbes’s account, the laws of nature are “essentially normative”.  Unlike theorems of geometry, which simply present us with explanations, laws of nature present us with imperatives.  They do not merely describe; they prescribe.  The second claim is that the definitivist interpretation of Hobbes’s ethics cannot account for this important feature of Hobbes’s system of thought.  The definitivist, that is, cannot explain why, on Hobbes’s account, moral science is normative in a way that the other sciences are not.  This claim can in turn be broken into two sub-claims.  The first sub-claim is that the only kind of explanation of this difference that would be consistent with the definitivist account would be one that explained the difference entirely in terms of the definitions of the various laws.  A definitivist explanation of the different status of geometrical and moral principles, that is, could appeal only to facts about customary linguistic usage and the logical properties of the terms used within them.  The second sub-claim is that no explanation of this sort can succeed.  If Murphy is correct about this, and if, as I am assuming, he is also correct that the orthodox interpretation can successfully account for this difference, then there is a substantial reason to prefer the orthodox interpretation to the definitivist one.
        I believe that the first claim of Murphy’s argument is correct: on Hobbes’s account, the laws of nature are essentially normative.  I also believe that the second sub-claim of the second claim is correct: a satisfactory explanation of the normative difference between ethics and geometry cannot be found merely by looking at the definitions of terms.  But it is not clear to me that the first sub-claim of the second claim should be accepted.  It is not clear, that is, that this sort of explanation of the difference between the two is the only one that a definitivist could offer that would be consistent with definitivism.  In principle, at least, it seems possible that a definitivist could instead point not to linguistic differences between the definitions used in, say, the laws of morals and the laws of physics, but rather to substantive differences between the subjects that are governed by these laws.  I am not prepared to argue that such a strategy will in fact succeed, but I would like to suggest that it is not obvious that it will not succeed, and so not clear that we are entitled to accept the first sub-claim needed to establish the second claim of Murphy’s argument.
        Consider, for example, the account of the difference between moral and physical laws developed by Kant in the Groundwork of the Metaphysics of Morals.   The account runs roughly as follows: everything in nature works according to laws.  When a rock falls, it does so according to the law of gravity.  But because it has no desires, and so, in particular, no desire to act against the law of gravity, it does not experience the law as a constraint on its behavior.  When a bird builds a nest, it does so according to the genetic laws that govern its species, but because it has no desire to act against these laws, it does not experience them as constraints on its behavior.  If there are perfectly rational beings, such as angels, the moral law will govern their behavior.  But since they will have no desire to act against the moral law, they will not experience the law as a constraint on their behavior.  Keeping promises, for an angel, would be like building a nest for a bird.  Each would be compelled to do so by the laws that govern their behavior, but neither would experience the activity as something that they were being required, against their will, to do.  As a result, the laws would best be understood as simply describing how beings of that sort behave, rather than as prescribing how they should behave.  Human beings, however, as Hobbes repeatedly stressed, are imperfectly rational beings.  We do have desires to act in ways that perfectly rational beings would not desire to behave.  It is for this reason that we experience the moral law as a constraint on our behavior and that for us it has the form of an imperative.  It is because we often desire to break our promises when we believe that we can get away with it, for example, that we experience the third law of nature as an imperative, commanding us to act in a certain way, rather than merely as a description of how we do, in fact, behave.
        On this sort of account, the question of whether a given set of laws will take a descriptive or imperative form is to be answered not simply by looking at the conventional definitions of the laws themselves, but also at the relation between the laws and the subjects that they govern.  If the subject governed by a particular law lacks conscious desires altogether, as in the case of a rock, then the subject will not experience the workings of the law in any way at all, and it will not make sense to address the law to the subject in any way at all.  If the subject governed by a particular law has desires but none that run counter to the law, as in the case of the bird or the angel, then the subject will not experience the law as a constraint on its behavior and it will not make sense to address the law to the subject in imperative form.  In both cases, it will make sense instead to address the law to those who seek to understand the subject’s behavior, and thus to present it in descriptive form.  It is only in the case where the subject does have conscious desires that run counter to the law that the subject will experience the law as a constraint on its behavior, and thus only in this case that it will make sense to address the law to the subject in imperative form.  If birds were frequently tempted to refrain from building nests, or if rocks had impetuous desires to roll up hills, then laws to the effect that birds build nests and that rocks roll down hills would appropriately be expressed to them in imperative form.  And if the point whose path traces a circle were tempted to stray from a constant distance from the circle’s center, the same would go for the principles of geometry.
        This kind of account of the difference between the normative status of moral and other scientific principles does make reference to facts about human beings in general, and about human desire in particular.  And so it might seem that this kind of account is not available to the definitivist, who rejects the claim that the truth conditions for the laws of nature make reference to facts about human desire.  But it is not clear that appealing to facts about human desire in this particular way contradicts what Murphy takes to be the claim that is, as it were, definitive of definitivism.  This is the claim that whether or not a particular class of actions is picked out by the laws of nature is a function of facts about customary linguistic usage only, not a function of facts about human desires, even if the facts about cutomary linguistic usage are themselves in some ways influenced by facts about human desire.  And it may be open to the definitivist to maintain that what makes it true rather than false that promise keeping is picked out by the laws of nature can be specified in purely linguistic terms, while still agreeing that what makes the picked out acts appropriately presented in imperative rather than descriptive terms is a function not just of the laws themselves, but of the relation between the laws and the subjects they govern.  I do not feel fully confident that such a strategy will succeed, but I also do not find it obvious that it will fail.  And for that reason, while I believe that Murphy has succeeded in defending the orthodox interpretation from the criticism Deigh aims at it, I am less confident that he has succeeded in overturning the alternative interpretation that Deigh would offer in its place.