Wolff's Philosophia Practica Universalis deals with a part of his philosophical system that wasn't nominally studied in his German writings and which was first presented as part of philosophy by Wolff's disciples. That is, the topic is the general part of practical philosophy, common to both ethics and politics, while we have German books only of ethics and politics. That said, many of the topics dealt here were included in Wolff's German ethics. In any case, universal practical philosophy is meant to be a study of the most general rules guiding free actions through knowledge of volition, when it is determined to some actions or non-actions. The aim of this part of Wolffian philosophy is also to offer motives for doing certain things and means for achieving those ends. In general, it should give criteria for deciding when some action should be or should have been done – that is, a heuristic for discovering truths of moral and politics.
An important question is obviously what to count as free action. The basic definition Wolff suggests is that free actions are not based on natural necessities, but on the liberty of soul. Wolff is here not trying to define or explicate human freedom – this should be the task of metaphysics – but merely takes the notion of freedom for granted. Basic distinction is that while sensuous appetites and aversions are natural, everything based on rational decision should be free. Although the distinction seems quite rigid, even in case of sensuous impulses there is some measure of freedom involved – we can e.g. freely move away from the vicinity of things causing certain sensuous appetites. Even such things as ignorance won't make actions unfree, if we just have had capacity to overcome this ignorance.
An important feature of free actions is that they can be evaluated, that is, they are good, bad or indifferent. For Wolff, the criterion of goodness and badness is dependent on the notion of perfection – actions promoting our perfection are good, while actions promoting our imperfection are bad. Wolff thinks also that these evaluations are natural in the sense that they are based on the essence of humanity – humans form a certain genus of entities, thus, they should act in a certain manner. The essence of humanity thus form the content of a natural law, which can thus be distinguished from all positive laws, authority of which is based on mere arbitrary decisions of human beings and their communities. Natural law works as a sort of general framework, on which all positive laws are based in the sense that the validity of the positive laws is instantly cancelled if they happen to contradict natural law.
Because the natural law is based on the essence of human beings, knowing natural law should be just a case of knowing what humans are like. Thus, natural law should in principle be possible to know by anyone. This was especially important conclusion in view of the topical question, whether atheists could be moral persons. Wolff concludes that they can be, at least partially. Natural law does have parts concerning God – human beings must work toward the glory of God. Yet, a significant part of natural law should be independent of such demands and thus be something that even an atheist could follow.
What then is a relation of God to natural law? God, as the creator of the whole world, has also decided that entities with the human essence exist. Thus, God might be called the instigator of natural law. In one sense, this doesn't really say much. True, following natural law will inevitably lead to happy and even blessed life, while transgressing natural law will in the long run lead to mere misery and torture. Yet, this is not so much because of God's particular punishments, but because making oneself perfect will also make one happy, while life geared toward one's imperfection will inevitably work against one's happiness. Although these rewards and punishments of good and bad actions are hence merely natural, nothing speaks against the possibility that God might decide to reward or punish people in a more personalised fashion according to the merits and demerits of their actions.
Following natural law leads thus to natural and perhaps even to special divine rewards. This still does not mean, Wolff says, that these rewards are the only motive for following natural law. Indeed, a virtuous person – that is, someone who has habituated herself to act according to natural law – will do good things just because he loves doing them, no matter whether she would get any tangible rewards for them. Similarly, a truly vicious person would be so engrossed with her perverted ends that she would not discontinue her wicked ways, even if she knew about the punishments awaiting her bad life.
Although Wolff thus accepts the power of habituation in forming one's moral outlook, the general tendency of his practical philosophy is rather intellectualistic. Thus, it is no wonder that according to Wolff, conscience is a form of judgement, instead of feeling. In other words, if one's conscience gives bad advice, this is not so much due to insufficient training or inner depravity of conscience, but more on a lack of good judgement. This does not mean that conflicts of conscience would not lead to any effect that we could feel – on the contrary, if we find out that our judgement has lead us astray, pangs of conscience will follow.
This first part of Wolff's general practical philosophy contains only quite theoretical principles that will be applied to more practical questions in the second book. The final topic Wolff manages to cover in this book is the question of responsibility. Generally speaking, Wolff thinks it is only free actions we can be responsible for. This does not mean that e.g. habits or deeds made in ignorance cannot be blamed or commended – habits can be followed with clear awareness, and ignorance might be something that we could have avoided. Although Wolff does not provide a general explanation what actions to blame and what to commend, he does mention what might be called second-level habits that are to be blamed or commended. Thus, diligence in following natural law is to be commended, while negligence of it is to be blamed.