One of the subtle but foundational changes that Newton introduced into physics was the extrinsicism of action, i.e. that physical action as such arises from extrinsic sources. The First Law is that action as such is from vis impressa, and absent this one has no action but only a uniform state of motion or rest.
Aristotle, by contrast, could accept all of Newton’s equations but would have balked at the claim that physical action was inherently extrinsic. The physical world is the totality of what arises by nature, which is an intrinsic cause of motion and of rest.
One clear difference is between the Newtonian and Aristotelian accounts of falling. Both agree that a fall is a tendency to a gravitational center, but in a Newtonian world this comes to be attributed to gravity, or a force between objects and outside of them, tugging them extrinsically. For Aristotle, by contrast, gravity is nothing but an intrinsic tendency of a body to seek a gravitational center, of which he (mistakenly) thought there was only one. Oddly enough, it’s hard to see how Newton avoids positing a superfluous entity that ends up acting from an intrinsic principle anyway, since if gravity is some sort of ghost tugging things to the center of mass it still seems to do so from an intrinsic principle. Tugging to a center is just what this ghost in inherently compelled to do because, well, that’s its nature. If it comes to that, why not just put an inherent nature in bodies and be done with it?
One answer might be that this would modify to some extent our theory of an experiment. Experiments act on things from without – this seems to be just what we mean by the experimental condition, i.e. something we introduce into a system from without that is not introduced into the control group. This gives the sense that the real magic occurs from what is introduced extrinsically. If something else isn’t introduced into a system from without, then how does anything in the system change?
Aristotle’s answer is that things are also being drawn into a system by the intrinsic natures of parts. So sure, water makes a difference in plant A that doesn’t occur in plant B if we deny it water, but this water is not just poured on the plant but also drawn into it.
The intersection between Newton and Aristotle modifies parts of both systems. Newton’s Laws seem like the best articulations we have of physical natures as such but they need to be recast as arising from the intrinsic natures of things. As already explained, what Newton called gravitas or heaviness is exactly what any Latin speaker would have thought it was – the inherent tendency of a body to move to some point or simply to fall. One needs no ghost to tug on the body. The more interesting modification comes in his notion of inertia. What Newton should have said is that the nature of the physical as such is to preserve action it receives from a higher order, whether that action makes the physical entity rest or move, and as an accidental effect of this preservation it resists attempts to alter it. Newton is in fact discovering the true nature of the physical as an instrumental cause of a higher order. He called it “inertia” but in fact it is subordination or instrumentality. What he called vis impressa traces back ultimately to the intrinsic nature of higher order beings like living things or angels moving natures according to impressions that the physical object preserves, bearing in mind that once a physical object is in motion it can also run into other objects and cause motions in them. Newton’s cosmos is, pace Naturalism, a system that moves partially of itself by its heaviness and partially in its openness to causes of a higher order, as when, for example, an experimenter introduces an experimental condition that he withholds from the control group. In fact, the essential instrumentality of physical things (or “inertia”) is exactly why experiments work, or even physics itself.
Just Thomism 