From owner-cse584-sp07-list@LISTSERV.BUFFALO.EDU Sun Mar 4 16:07:04 2007 Received: from ares.cse.buffalo.edu (ares.cse.Buffalo.EDU [128.205.32.79]) by castor.cse.Buffalo.EDU (8.13.6/8.12.10) with ESMTP id l24L743S015275 for ; Sun, 4 Mar 2007 16:07:04 -0500 (EST) Received: from front3.acsu.buffalo.edu (upfront.acsu.buffalo.edu [128.205.4.140]) by ares.cse.buffalo.edu (8.13.6/8.13.6) with SMTP id l24L70Km039028 for ; Sun, 4 Mar 2007 16:07:00 -0500 (EST) Received: (qmail 10202 invoked from network); 4 Mar 2007 21:07:00 -0000 Received: from mailscan1.acsu.buffalo.edu (128.205.6.133) by front3.acsu.buffalo.edu with SMTP; 4 Mar 2007 21:07:00 -0000 Received: (qmail 8402 invoked from network); 4 Mar 2007 21:07:00 -0000 Received: from deliverance.acsu.buffalo.edu (128.205.7.57) by front2.acsu.buffalo.edu with SMTP; 4 Mar 2007 21:07:00 -0000 Received: (qmail 20040 invoked from network); 4 Mar 2007 21:06:48 -0000 Received: from listserv.buffalo.edu (128.205.7.35) by deliverance.acsu.buffalo.edu with SMTP; 4 Mar 2007 21:06:48 -0000 Received: by LISTSERV.BUFFALO.EDU (LISTSERV-TCP/IP release 14.5) with spool id 3623673 for CSE584-SP07-LIST@LISTSERV.BUFFALO.EDU; Sun, 4 Mar 2007 16:06:47 -0500 Delivered-To: cse584-sp07-list@listserv.buffalo.edu Received: (qmail 15795 invoked from network); 4 Mar 2007 21:06:46 -0000 Received: from mailscan3.acsu.buffalo.edu (128.205.6.135) by listserv.buffalo.edu with SMTP; 4 Mar 2007 21:06:46 -0000 Received: (qmail 13190 invoked from network); 4 Mar 2007 21:06:45 -0000 Received: from castor.cse.buffalo.edu (128.205.32.14) by smtp1.acsu.buffalo.edu with SMTP; 4 Mar 2007 21:06:45 -0000 Received: from castor.cse.Buffalo.EDU (rapaport@localhost [127.0.0.1]) by castor.cse.Buffalo.EDU (8.13.6/8.12.10) with ESMTP id l24L6jvk015267 for ; Sun, 4 Mar 2007 16:06:45 -0500 (EST) Received: (from rapaport@localhost) by castor.cse.Buffalo.EDU (8.13.6/8.12.9/Submit) id l24L6isB015266 for cse584-sp07-list@listserv.buffalo.edu; Sun, 4 Mar 2007 16:06:45 -0500 (EST) X-UB-Relay: (castor.cse.buffalo.edu) X-PM-EL-Spam-Prob: : 7% Message-ID: <200703042106.l24L6isB015266@castor.cse.Buffalo.EDU> Date: Sun, 4 Mar 2007 16:06:45 -0500 Reply-To: "William J. Rapaport" Sender: "Philosophy of Computer Science, Spring 2007" From: "William J. Rapaport" Subject: VALIDITY AND INVALIDITY To: CSE584-SP07-LIST@LISTSERV.BUFFALO.EDU Precedence: list List-Help: , List-Unsubscribe: List-Subscribe: List-Owner: List-Archive: X-UB-Relay: (castor.cse.buffalo.edu) X-DCC-Buffalo.EDU-Metrics: castor.cse.Buffalo.EDU 1029; Body=0 Fuz1=0 Fuz2=0 X-Spam-Status: No, score=-2.0 required=5.0 tests=AWL,BAYES_00,SUBJ_ALL_CAPS autolearn=no version=3.1.7 X-Spam-Checker-Version: SpamAssassin 3.1.7 (2006-10-05) on ares.cse.buffalo.edu X-Virus-Scanned: ClamAV 0.88.6/2722/Sun Mar 4 13:06:42 2007 on ares.cse.buffalo.edu X-Virus-Status: Clean Status: R Content-Length: 1570 A student writes: | Just for some clarity, will a student find himself deducted 9 points | for failing to include MP's in the situation where some premises may be | true and some may be false, but the conclusion is false. From my | understanding that would be a valid argument because some of the | premises are false and the conclusion is false, therefore the argument | is valid. Is this approach correct? First, the grading rubric for PP2 isn't the same as for PP1, so there's no "9 point" issue. If you didn't see the PP2 rubric, go to: http://www.cse.buffalo.edu/~rapaport/584/S07/EMAIL/20070228-PosPaper2.txt Second, an argument is valid if and only if it is necessarily "truth-preserving", i.e., if & only if it's impossible for all premises to be true but the conclusion false. Note that this has nothing at all to do with whether any of the premises actually *are* true or false; it's a "what if" kind of situation. So you can have an argument with false premises and a false conclusion that's valid, and you can have one like that that's invalid. Here's a valid one: All cats are fish. All fish can fly. Therefore, all cats can fly. Here, everything's false, but the argument is valid. Here's an invalid one: All cats are fish. All cats can fly. Therefore, all fish can fly. Again, everything's false; moreover, the argument is invalid. Missing premises can *sometimes* make an invalid argument valid, as we saw in PP1. In other cases of invalid arguments, no missing premises will help; for an example, look at the invalid argument above.