Lawyer: A body of circumstantial evidence is like a rope, and each item of evidence is like a strand of that rope.
Conclusion (thus)
Even if a few items of a body of circumstantial evidence are discredited, the overall body of evidence retains its basic strength.
Evidence
A body of circumstantial evidence is like a rope, and each item of evidence is like a strand of that rope.
Just as additional pieces of circumstantial evidence strengthen the body of evidence, adding strands to the rope strengthens the rope.
And if one strand breaks, the rope is not broken nor is its strength much diminished.
Evaluate
The argument assumes that the impact of losing a strand of a rope on the overall strength of that rope and the impact of losing an item of circumstantial evidence on the overall strength of the body of evidence is similar.
When authors lean on analogies, we try to object to those arguments by pointing out salient differences.
How would we potentially argue that ?
Maybe we could say that those few items might be really important to the case, or maybe they are important to other pieces of evidence (i.e. if you lose certain pieces, you might simultaneously then lose other pieces that were contingent on that piece).
These would point out ways in which a body of circumstantial evidence is disanalogous to the strands of rope: the strands of the rope are generally similar in quality and strengthening-effect and don't rely on other strands, whereas some pieces of circumstantial evidence might have more strengthening power than others and might rely on other pieces of evidence.
Goal
Look for an answer pointing out a potential difference between bodies of evidence and a rope made up of strands, so that we can argue that it's possible that losing a few pieces of circumstantial evidence could really mess with the strength of your case.