In every world, Crusius notes, things interact by moving, thus movement is a topic that should be studied in cosmology. More precisely, he continues, the topic of movement can be approached with three questions: what is movement, how it can be measured and what laws govern it. Of these three, Crusius explains, the first one is clearly cosmological, while the second one was dealt in ontology. The third one, Crusius concludes, is partially cosmological, because some laws of movement can be deduced from the very essence of movement, although some laws of movement must be proven empirically and thus belong to physics.
Crusius then defines movement as a state of a substance that changes its place. Its opposite is rest or a state where a thing does not change its place. Crusius notes also that some movement is only apparent, when a thing does not change its absolute place, but only its place in relation to some other thing. True change, where a thing changes its absolute place, he further divides into external change, where the whole thing is moving, and internal change, where some actual parts of a thing change their places so that their positions in relation to other parts of the whole are changed. Crusius points out that these two classes are not mutually exclusive, since there can occur situations where a thing moves, while its parts change their place in relation to one another.
What then could move in these different ways? Crusius notes that a simple substance cannot at least move internally, since it doesn’t have any actual parts. Then again, infinite things (or from Crusius’ perspective, God) cannot move externally, because there is nowhere where such an infinite thing could move as a whole. Furthermore, he adds, everything that is finite, can move at least externally, whether it is simple or complex.
Crusius divides external movement into total external movement, where the whole substance moves completely from one location to another, and partial external movement, where actual or ideal parts of the thing change their place: once again, the two classes are not mutually exclusive. While the notion of total external movement seems clear enough, it might be difficult to see how partial external movement differs from internal movement. One example of a partial external movement that is not internal nor total, Crusius says, is such where a simple substance grows and thus its ideal parts change their places, since this involves no change of actual, distinct parts of the substance. Another example would be rotation of a sphere: the sphere as a whole does not go from one location to another and the relative positions of its parts remain the same, but the parts of the sphere do change their absolute positions.
Crusius derives some simple consequences from his definition of movement. Movement, as he sees it, has always a definite direction, and indeed, a start and end point. It also has certain characteristics: velocity and strength by which it withstands resistance. Furthermore, Crusius notes that movement of a complex substance is defined by the movement of its parts.
Movement, Crusius emphasises, is a positive change and thus requires a positive cause. Rest, on the other hand, is for Crusius just a lack of movement and does not therefore require any cause. In other words, a thing rests, Crusius says, if it has no reason to move, and if the cause of the movement vanishes, the movement must also cease. A direct consequence of this is that increase in the velocity of movement requires a similar increase in the cause of the movement. Crusius also thinks that change of the direction of the movement must also have a cause, which would make the Epicurean idea of atoms swerving without a reason ridiculous.
Because motion always requires some cause, Crusius continues, state of movement cannot be indifferent to the matter. By this Crusius means that a moving cause has to at first overcome an inherent resistance in moving a piece of matter. This inherent resistance is, of course, inertia. More precisely, Crusius calls it metaphysical inertia, distinguishing it from physical inertia, where the resistance is not just an inherent property of matter, but also involves a force, although one that is, as it were, dead, that is, suppressed by the moving force. Beyond inertia, motion can be resisted also by a living force, that is, a force that truly can resist the moving cause.
Crusius notes that a finite cause of movement cannot really affect a thing more than the thing resists the movement. Of course, the cause can have more force, but it only uses as much force as is required for overcoming the resistance. Thus, Crusius thinks he has justified the law of action being equal to reaction. Although a non-empirical proof, Crusius clarifies, we still require empirical observations to determine how much a finite cause acts at a given situation.
It is an essential feature of substances that differ from God, Crusius says, that they cannot penetrate one another. Thus, when a finite substance, whether matter or spirit, tries to occupy the same place as another finite substance, it will drive away the other substance, that is, pushes it. Crusius adds that pushing is just an existential effect, in other words, it doesn’t require any force, but the mere presence of one substance trying to occupy the place of the other.
Pushing a substance makes it move: Crusius calls this a communicated movement. Indeed, he says, communicating movement is the only way finite substances can affect one another. Series of communicated movements cannot go on forever, he immediately adds, and such series cannot all derive from God’s miracles, because that would be against the purpose of the world. Thus, Crusius argues, there must be some finite substances that can move their own substance through their inner activity - such movement he calls original. The inner activity causing original movement can be constant or conditional striving, inherent to some elements of material things, or it can be free willing. Because even the activity of elements is ultimately derived from God, Crusius concludes, all movement is generated by some spiritual activity that is not movement.
Communicated movement, Crusius notes, need not always be just an effect of the impenetrability of finite substances, but can also involve an inner activity of a substance. He is thus against the Cartesian idea that interactions of material things would have to be explained solely through geometric properties: God can give material things some inner activities. Crusius faces the possible objection that such activities are what were disparagingly called occult qualities by noting that we can know such activities as well as finite creatures can, when we can distinguish them from one another and deduce their existence from their effects.
Crusius progresses then to describe several rules involved in the communication of movement, such as parallelogram rule and behaviour of elastic substances. I will not follow him to these details, but I shall take a look at a few conundrums concerning movement that Crusius considers. First of these involves the question of the quantity of movement in the world: is it always constant? Crusius’ answer clearly has to be negative, because this would preclude the possibility of spirits to interact with the world. He even denies the weaker assumption that the world would have a constant amount of moving force, because spirits should be able to choose how strongly they move other things.
Another conundrum concerns the question whether all the matter in the world is moving constantly. A reason for upholding such an opinion would be that a constant movement is required for explaining why shapes of things remain stable: without the constant movement of the surrounding matter, a thing could just change willy-nilly its shape. Crusius does not find this argument convincing. The shape of a complex thing is determined by the shape and position of its parts, while the shape of a simple thing is either chosen by God, with or without any reason, or caused by themselves or by external forces - where is the need for movement here?
The answer to the question, Crusius concludes, belongs to physical, not metaphysical cosmology. He does state that the ultimate limits of the world cannot move and similarly all things that God has determined to rest. Other things, then, probably at least strive to move, whether through their own inherent activity or through being spurred to movement by things outside them.
Finally, Crusius ponders the question whether movement of one material thing necessarily sets all other material things in motion. He notes that if the world contains spaces void of any finite things, movement of one thing could go through this void without communicating movement to other things. Furthermore, he continues, even if there is no void, certain material things could also just switch places without affecting other material things.
Crusius also thinks that these arguments disprove the idea endorsed often by Wolffians that from state of any substance in the world could be determined the state of all other substances. This proposition of Wolffians was explicitly based on the supposed continuous causal nexus of all the parts of the world, where movement of one piece would eventually affect all the other pieces. If this nexus fails, as Crusius deems very possible, the states of the substances in the world would not be as closely interlinked, although they would be really connected.
