The naturalistic fallacy claims that an ought-statement cannot be derived from an is-statement. If true, this fallacy would destroy the traditional Thomistic conception of natural law, which argues precisely from is-statements about human nature to moral ought-statements. There are two ways to overcome the fallacy. The first is taken by the proponent of the “new natural law,” the so-called new natural lawyers. They argue that natural law is not based on is-statements about human nature, teleology, or divine command. Rather, natural law is based on the first principles of natural law that are per se nota. These principles have a prescriptive force, so any practical syllogisms derived from them would not be subject to the naturalistic fallacy. Steven Jensen, however, takes the second way and argues that the naturalistic fallacy is not a fallacy at all. It is possible to derive an ought-statement from an is-statement. Moreover, he argues that Aquinas holds the same position. Jensen lucidly and systematically argues that the root error of the naturalistic fallacy is a complete separation of speculative and practical knowledge. Following Aquinas’ lead, Jensen argues that there are two types of partially speculative and partially practical knowledge between the purely speculative and the purely practical. These two types of cognition, which Jensen calls materially practical and virtually practical, provide the bridge from purely speculative is-statements to purely practical action.
Good action, Jensen argues, begins with a purely speculative knowledge of inclinations. These inclinations are either natural (as the intellect desires to know the truth) or cognitive (as in the passions and the will). Knowledge of our natural inclinations allows us to know the eternal law and our participation in it, which Aquinas calls the natural law.
From this speculative knowledge, we come to materially practical knowledge of what is good for us. Our inclinations reveal a potency in us that needs completion. We desire only certain objects because they have some inherent attributes necessary for our completion. These desired objects are recognized as good precisely because they complete the potency. We might know the good as an effect before we recognize it as the end of an inclination, but we only know the good as good when we are aware of the inclination. Once the good is discovered, our knowledge is materially practical.
The knowledge of what is good for us becomes virtually practical when we further consider the how-to knowledge of ought-statements. At the previous stage, we reason from effects to causes, but at this stage, we reverse course and reason from causes to effects. We shift from considering what is necessary for this effect to occur to considering what is necessary in the cause for the effect. This shift introduces the term “ought” because the end imposes a hypothetical necessity on the cause. Given that a builder has the end of a completed house, he ought to lay a firm foundation. Hypothetical necessity seems insufficient because someone may opt out of the end or prefer a different one. Jensen argues that this objection errs in claiming that an individual's will is ordered to a merely private good, not a common good. Since the will is ordered to a shared good, those who opt out of that good are, in effect, opting out of a good that they naturally desire and cannot fully stop desiring.
Finally, our knowledge becomes fully practical when the virtually practical knowledge “I ought” is joined to desire. Before desire, we want to perform the activity of a means to an end, but until the will recognizes the means as good (all things considered), it will not desire them, and we will not perform the action. Unfortunately, although he specifies several types of desire, Jensen does not specify exactly what sort of desire he means here.
Jensen admirably defends the position that our knowledge of first principles is speculative and that we can and do move from that speculative knowledge to action. He argues for this position in light of the objections from the new natural lawyers, acknowledging what is true in their objections, and responding to their criticisms. This book is a clear account of natural law. I strongly recommend it for all moral philosophers and theologians and for use in a graduate classroom.
