Philosophy of Science: Nancy Cartwright

I recently finished reading Do the Laws of Physics State the Facts by Nancy Cartwright (not the voice actor for Bart Simpson) in Philosophy of Science: The Central Issues Second Edition and am just as confused as I was yesterday :)) which is always fantastic. We have yet to cover it during my philosophy lecture but what I can infer from the reading is Cartwright’s bluntness about the laws of gravitation and ultimately physics not stating facts. She did not “beat around the bush” and tackled the problem head-on. What was so intriguing was she was not trying to debunk the facticity of physics entirely, but she claimed the usefulness of physics and others alike. Talking about the laws of gravitation: “Does this law truly describe how bodies behave? Assuredly not.” She refers to the facticity of the law of gravitation with the law of energy because they are contradictory. Which makes sense. 
Cartwright than shortly introduces how vector addition introduces causal powers because when “adding” forces it’s unrealistic or a “metaphor.” When talking about something as imperative as “forces” and gravitation (reasons for why and how an innumerable amount of things are today), we cannot add them like 1+1. This idea of vector addition “is a nice one,” but not something defining the facts of nature. See the source image
Cartwright then introduces Mill, the rebuttal, and says why he would perhaps deny these claims of the un-facticity of vector addition. “Mill would deny this. He thinks that in cases of the composition of causes, each separate effect does exist --- it exists as part of the resultant effect, just as the left half of the table exists as part of the whole table” (Cartwright 876). Cartwright disagrees, these entities Mill is talking about are more temporary than what he is referring them to be. Without delving too much into the issue, it’s important to note that “the laws of physics are to explain how phenomena are brought about, they cannot state facts.”