Informal Logic

I've written a few times about formal logic, and its importance on the LSAT, GRE, GMAT, and SAT, but informal logic is also very important to those tests. The difference between formal and informal logic is that formal logic is more mathematical, while informal logic is more rhetorical; that is, in formal logic we use symbols and shorthand to help us isolate variables (such as the truth, falsehood, or unknowability of certain premises), while informal logic is based upon generally agreed upon rules for what does and does not constitute a persuasive argument.


The difference in these two skills is probably most pronounced for LSAT students, since the logic games section relies heavily upon formal logic skills, while the critical reading and logical reasoning sections depend mostly upon informal logic. For instance, strengthen, weaken, assumption, paradox, principle, and especially logical flaw questions hinge on the student's ability to recognize and articulate violations of the rules of informal logic. Flaw questions, especially, can be tough for non-native English speakers, because the answer choices often come off as vague and confusing. One reason for this is that every answer choice--even the wrong ones--articulates some logical flaw; only one answer, however, will be the correct flaw.


As I always tell my students, however, many flaw questions also have a hidden "cheat code" built into them: Flaws often have specific names (sufficiency vs. necessity, unexamined alternatives, ad hominem, etc.). To accurately describe the flaw, the testmakers almost have to put the flaw's name in the correct answer choice. Hence, if you learn to recognize the flaw when you see it, you can often just scan the answer choices for a single keyword.


If you're interested in reading more about informal logic, I recommend the book Informal Logic: A Pragmatic Approach by Douglas Walton. The author analyzes actual arguments which have been made in the public forum, and uses clear, simple explanations to discuss each one. Also, the book moves nicely from simpler arguments to more complex ones, and points out how to recognize and articulate common logical fallicies. Whether you're a young student bound for college, or a more mature adult ready to start lawschool or grad school, informal logic is an invaluable tool that will sharpen your mind and make you less vulnerable to specious rhetoric.

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・【Study Like a (Good) Dog!】2010.04.14 (Wed)