If my account of normative normality is roughly correct, then we can see that the human good is something that belongs, both semantically and metaphysically, to the domain of the objects of theoretical knowledge (in contradiction to proposition 5 of the NNL). It is possible for God or an angel to know the human good from a purely third-person, scientific perspective, by simply investigating the human essence and its causal powers, and their interdependency.
But can we human beings know the human good in this way, or must we know it exclusively in first-person, practical mode of knowledge (as proposition 4 claims)? I don’t want to deny that we have such a distinctively practical way of knowing the human good. And I would agree that this practical knowledge is our primary mode of access to those basic goods.
In Nicomachean Ethics Book I, Aristotle asserts that both being well brough-up and having extensive experience are necessary conditions for grasping the principles of ethics. The agentive perspective that is required for complete practical knowledge of human goods is not innate. It must be developed over time. All the theoretical knowledge in the world will do the human person no practical good at all without a substantial foundation of practical knowledge.
However, none of this entails that theoretical knowledge of the good can make no contribution at all to our practical knowledge. I see at least three ways in which theoretical knowledge can contribute to our practical knowledge. First, as a corrective. Our practical apprehension of the good is not infallible. We can practically believe in a false value. For example, many people have believed that managing slaves is a basic human good. Theoretical knowledge of human nature can defeat such errors of practical reason.
Second as a supplement. We can fail to apprehend genuine human goods in a practical way. For example, C. S. Lewis admitted in Mere Christianity that he lacked a practical knowledge of the value of loving and enjoying children, and his theoretical knowledge of human nature enabled him to recognize this lacuna in his practical makeup. This kind of supplemental use of reason may not be sufficient to make us virtuous. A virtuous person must grasp the good in a fully practical, first-person way. However, the supplemental use of philosophical anthropology could give us true opinion about what is good, enabling us to achieve what Aristotle labels ‘continence’ (ἐγκράτεια). Continence may fall short of virtue in reliably guiding us to the good, but it can still be valuable nonetheless.
Finally, as an aid to growth in practical knowledge. To acquire practical knowledge, we must acquire virtue. Acquiring virtue requires that we act consistently in a way that emulates virtuous action. In doing so, we can rely on theoretical knowledge of the good. So, for example, Lewis might have gained some practical knowledge of the value of children by imitating the actions of those with the appropriate knowledge.
The Analytic Thomist 