What do the joints signify in the metaphor "carve nature at the joints"?

_{I first encountered this metaphor in the 2nd sentence of p 267 , Think: A Compelling Introduction to Philosophy (1 ed, 1999) by Simon Blackburn. If it pertains to my question below, please ask me to quote it.
Source: p 309. Women, Fire, and Dangerous Things by George Lakoff}

[...] On this view, a conceptual system can succeed for fail to fit well, that is, to "carve nature at the joints." It can choose different joints to carve at, that is, it can con ceptualize different aspects Of reality. In addition, conceptual systems can vary in their "fineness of grain," that is, they can carve nature into big chunks or small artful slices: as Whorf puts it, with a "blunt instrument" Or a "rapier." But a conceptual system cannot create new joints, because objectivism assumes that all the joints are given ahead of time, ob- jectively, once and for all. A conceptual system is accurate if it always finds joints in nature (though it certainly won't find all of them) and inac- curate if it misses the joints and hits a bone or nothing at all.

Even if I correctly inferred 'joints' to signify the parts of nature, the noun 'joints' implies a sense of organised structure or synthesis, and so appears a strange choice for referring to something as pure and spontaneous as nature. So what have I neglected?

Decoding metaphors can be tricky. It's very important to distinguish the tenor of the metaphor from its vehicle. The former is the actual thing being described. In the case of your example, it's nature. The vehicle is the figure which the reader is invited to imagine, and to visualize in analogy to the tenor.

In the text cited here, to "to carve at the joints" refers to a method of dismembering a piece of meat - likely the carcass of a fowl or even (if one is hungry and thinking of a medieval feast) something considerably larger. That the carving is associated here with a "rapier" reinforces the image of whacking great chunks from the carcass.

I might suspect that you may be overthinking the metaphor by reading aspects of the tenor back into the vehicle, a reversal that's certainly easy to experience in the midst of a discussion of B.L. Whorf and conceptual frameworks.