The primary classification of things in metaphysical treatises has long been that of substances and accidences and Baumgarten's Metaphysics makes no exception. Substances are things that can exist without being attached to something else, while accidences have to exist in something else, namely, in substances. Furthermore, Baumgarten adds, accidences are not just something externally connected to a substance, but a substance must contain some reason why such accidences exist within it. In other words, substance is a force that in a sense causes its accidences – if completely, they are its essentials and attributes, if partially, they are its modes.
Now, substances with modes are variable or they have states, which can change into other states. Like all things in Baumgarten's system, these changes also require grounding in some forces. Changes effected by forces are then activities of substances having these forces. Such activities might be connected with changes in the active substance itself, but they might also link to changes in other things: these other things then have a passion. In latter case, the forces might act alone to produce a certain effect and then we speak of real actions and passions, or then the passive substance also has some activity at the same time as it has passions, and then we speak of ideal actions and passions. The division of real and ideal actions and passions is of importance in relation to Baumgarten's thoughts about causality.
Because all substances have forces, all of them have also activities – if nothing else, then at least activities towards themselves. Furthermore, activity does not define just the essence of substances, but also their mutual presence – substances are present to one another, Baumgarten says, when they happen to interact with one another.
Baumgarten divides substances, in quite a Wolffian manner, into complex and simple substances (Baumgarten does admit that we can also have complexes of accidences, but these are of secondary importance in comparison with complexes of substances). Not so Wolffian is Baumgarten's endorsement of Leibnizian term ”monad” as the name of the ultimate simple substances. Rounding up the division of substances is the division of simple substances into finite and infinite substances, in which infinite substance has all the positive properties in highest grade and thus exists necessarily and immutably – this is something we will return to in Baumgarten's theology – while finite substances change their states and have restrictions.
This concludes Baumgarten's account of the substances or primary entities of the world, and like with Wolff, we can already discern the outlines of the three concrete metaphysical disciples. But before moving away from ontology, we still have to discuss Baumgarten's account of basic relations of entities.