What, then, is the status of these precepts? The short answer is that they are not any kind of knowledge identified by Aquinas; they are what Grisez calls prescriptive knowledge.1 Prescriptive knowledge seems to be knowledge with a moving force to it, but not the moving force of the will; rather, the knowledge by its very nature has a moving force. Indeed, this knowledge is prior to the act of will and causes the act of will. This prescriptive knowledge in no way depends upon some prior end; rather, it determines the end. Since this prescriptive knowledge is practical, it does not simply reflect the way the world is, but rather precedes the world and ultimately shapes the world. Practical propositions, on this view, are true by preceding their realization.2
This state, says Grisez, is expressed in the Latin by the gerundive, translated as "is to be." In contrast, Aquinas claims that the gerundive is in the indicative mood.
Unfortunately, Grisez never clearly defines what this is-to-be status means, nor does Finnis or Lee, nor any other new natural law theorist. As is usually the case when "ought" is left undefined, we get nothing more definite than the hint of a categorical imperative, a mysterious knowledge that binds by its very nature. Practical reason itself becomes the lawgiver.
Prescriptive knowledge imposes a kind of odd necessity. It is not the necessity of commanded efficient causality. Nor is it a hypothetical necessity depending upon the desires of the individual, for it precedes the desires of the individual. Prescriptive knowledge attempts to walk a fine line. Its necessity does not arise internally from the individual’s desires, but neither does it arise externally from someone else’s command. Rather, it arises internal to the individual – from his knowledge – but external to his desires.
This balancing act, however, still does not explain the necessity found in a moral ought. It is neither the necessity of efficient cause, formal cause, nor hypothetical final cause. Once again, the moral ought escapes our grasp. As Foot says concerning those who use an "emphatic use of ought’," it seems "they are relying on an illusion, as if trying to give the moral ought’ a magic force.3
In the end, we are left with no explanation of the necessity behind a moral ought. Suarez substituted the desires of the individual for the command of God; Grisez substituted the command of God with prescriptive reason. In both cases a special power of binding is asserted but not explained. Should we follow Philippa Foot, then, and simply abandon the "moral ought," affirming that all ought-statements are hypothetical?4 Is the will bound by nothing beyond itself and its own ends? Perhaps the moral ought is, as G. E. M. Anscombe says, "A word containing no intelligible thought."5 We have yet to consider a final possibility. Can nature provide a source for the necessity of the moral ought?

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Grisez’s prescriptive knowledge is a hybrid, a mysterious knowledge that seems to warrant an entirely new faculty, a faculty of practical reason, separate and distinct from our faculty of grasping the world around us.1
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1 27. Paterson ("Non-Naturalism," 177) claims that Finnis gives verbal assent to Aquinas’s assertion that practical intellect and speculative intellect are a single faculty, but in fact Finnis’s view separates them into two faculties. A similar accusation is made by Doug¬ las Flippen in "Natural Inclinations," 306.

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1 39. Grisez, "First Principle," 194. In contrast, Rhonheimer ("Cognitive Structure," 178) insists that practical and prescriptive knowledge is a command.
2 40. Grisez, "Practical Principles," ii6.
3 42. Foot, "Hypothetical Imperatives," 167.
4 43. Later, Foot claims to have abandoned this position, but her later view does not seem essentially different; see Foot, Natural Goodness, 60-61.
5 44- See Anscombe, "Modern Moral," 32.