[gerard.casey]

David,

Switching them around as you did:

BIC
CAD
therefore BAD

would give you an invalid syllogism as it would fail rule 6. As you noticed, it also fails rule 2! Are you on the right track? Yes. You put the essential point very well – using your language, you can go from large to small (validly) but not from small to large.

Here’s an example of an invalid syllogism that fails only rule 6:

PEQ
QIR
therefore POR

If you run the six rules over this, you’ll see that if passes the first 5 but fails no. 6.

If you start a new thread, email me at gerard.casey@ucd.ie to let me know.

Keep up the good work.

Best wishes,

GC