Here's a passage that helps me make sense of a pretty significant division in contemporary philosophy. From Beta 3:
The issue here is whether genuine knowledge of thing comes by knowing the genera (τὰ γένη) or by analyzing things into their parts (ἐνυπαρχόντων) and how they connect. Most obviously, Aristotle takes a stand in favor of knowing the genus of a thing as the proper approach.
Apart from the great difficulty of stating the case truly with regard to these matters, it is very hard to say, with regard to the first principles, whether it is the genera that should be taken as elements and principles, or rather the primary constituents of a thing; e.g. it is the primary parts of which articulate sounds consist that are thought to be elements and principles of articulate sound, not the common genus--articulate sound; and we give the name of 'elements' to those geometrical propositions, the proofs of which are implied in the proofs of the others, either of all or of most. Further, both those who say there are several elements of corporeal things and those who say there is one, say the parts of which bodies are compounded and consist are principles; e.g. Empedocles says fire and water and the rest are the constituent elements of things, but does not describe these as genera of existing things. Besides this, if we want to examine the nature of anything else, we examine the parts of which, e.g. a bed consists and how they are put together, and then we know its nature.
To judge from these arguments, then, the principles of things would not be the genera; but if we know each thing by its definition, and the genera are the principles or starting-points of definitions, the genera must also be the principles of definable things. And if to get the knowledge of the species according to which things are named is to get the knowledge of things, the genera are at least starting-points of the species. And some also of those who say unity or being, or the great and the small, are elements of things, seem to treat them as genera.
But, again, it is not possible to describe the principles in both ways. For the formula of the essence is one; but definition by genera will be different from that which states the constituent parts of a thing. (998a20-998b14)
To break a thing into its parts, so as to understand the components and how they all fit together, is useful indeed. I've actually built furniture... in order to make a table, one early step is exactly to figure out how many separate parts it has, what their dimensions and shape need to be, and how they will fit together and stay that way. Exploded-view drawings and yard lists and cut lists and specs for final dimensions all come into play; the more detailed, the better.
Important as it is for understanding the details of the form of the thing, this process doesn't tell me anything about what the assembled table is, however. It's four legs with a top and an apron, maybe a stretcher for extra stability. I can understand exactly what each part looks like, how they go together, and the material I'll have to use. Add in my additional knowledge of furniture-making tools and techniques, awareness of where I might need help from someone else (maybe an upholsterer) and the analytic approach actually covers three of the four causes: Formal, Material, And Efficient!
Again, though, these three causes do not help me understand the use for the table, nor why it has this form or needs to be made out of this material or why these techniques of making it are appropriate. All of that additional knowledge seems to come from somewhere else. In the terms Aristotle presents here, all of that is apparently involved in knowledge of the genus of the table.
We all know what Aristotle means by genus, of course... genus and species, taxonomy, categorization and classification, all that. A "coffee table" is a species of the genus "table," or perhaps "furniture." (There are levels of generality... Aristotle later becomes very concerned whether being is a genus, the one to which everything belongs.) Most people probably tend to think of this as a system of organizing knowledge. Except there's something else going on here.
The lexicon indicates that "genus" isn't primarily a logical concept, and maybe it was never used that way until Aristotle did so. What we know the word "genus" to mean (thanks to Aristotle) was for him almost a metaphorical use of the word. In Greek, it generally (pun intended) referred to the origins of people or things, to their historical genesis and development over time. Aristotle's definition of "genus" in Delta 28 seems to be the main source for the definition in Liddell and Scott, in fact: Four definitions, the first two having to do with the historical origins of the persons or things (its genealogy), one concerning geometrical figures, and the last purely a matter of logical definition and classification.
This is not surprising, since Aristotle was a biologist, the son of a physician, and was acutely aware of the historical connections among life forms. His concept of genus as the classification which represents the actual relationships among species would be entirely modern, if he had known about evolution. That's a pretty big "if," of course.
Back to the point: when Aristotle considers whether knowledge according to genus or according to analysis is the better approach, he's to some degree anticipating aspects of the 20th century conflict between linguistic/logical analysis (analytic philosophy) on the one hand, and genealogy (Continental, e.g. Nietzsche and Foucault) or genetic method (pragmatism, especially Dewey) on the other. To know a thing--this table I've built, or the contemporary concept of "substance"--is to know its origins, and the relations it (and its ancestors) bear to other beings across time. So, this table is in the Craftsman style, based on a Gustav Stickley design, 18 x 18 x 16 inches high, made of cherry wood harvested from a tree on the farm down the road, cut and assembled by me with traditional pinned mortise and tenon joints, and so on. Likewise, this concept of substance is.... we have to tell the story of its origins. Ahistorical analysis, no matter how thorough, doesn't quite tell us enough.
Going on too long here, so, summing up. First, Aristotle says knowledge by genesis, not by analysis, is genuine knowledge. And second, his account of knowledge by genus makes sense of his emphasis on the four causes--the "causes" are the principles of genesis of a thing.