Hi Mark!
To see why the language, "thinking machines closely modeled on the brain are also likely to fail" is a subsidiary conclusion supporting the main one, let's break apart the argument that the question asks about:
Important does not mean essential. After all, no flying machine closely modeled on birds has worked; workable aircraft are structurally very different from birds. So thinking machines closely modeled on the brain are also likely to fail. In developing a workable thinking machine, researchers would therefore increase their chances of success if they focus on the brain’s function and simply ignore its physical structure.
- P1: Structural similarity to the brain may be important, but this does not mean essential, for thinking
P2 (analogy): No flying machine closely modeled on birds has worked; workable aircraft are structurally very different from birds
C: So thinking machines closely modeled on the brain are also likely to fail.
P1: Thinking machines closely modeled on the brain are also likely to fail.
C: In developing a workable thinking machine, researchers would therefore increase their chances of success if they focus on the brain’s function and simply ignore its physical structure
The "So thinking machines..." language is a conclusion because you can ask--
why are those modeled on the brain likely to fail? The answer is the previous two sentences--structural similarity to the brain is not necessary for thinking, and sometimes designing something to to achieve the same function--flight--requires the structure of planes to differ from the structure of the original, birds.
We know that it's the
subsidiary conclusion, because we can also ask why regarding the last sentence, "In developing a workable..."
Why would they increase their chances of success in research by focusing on the brain's function and ignoring its structure? The answer to this is that research modeled on the structure of the brain is "likely to fail" (so they should instead focus on brain function). The likely-to-fail language is what followed from the initial premises, and is itself also a premise that is used in support of the main conclusion.
