Introduction to Philosophy (Fall '09) - class#2 for website

Introduction to Philosophy (Fall '09) - class#2 for website...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Organizational Information Discussion sections meet this Friday (tomorrow) The Power Point presentation from my first The lecture is now up on the website and the Power Point presentation for this class will also be on the website later today. the http://courses.umass.edu/phil100-pgraham http://courses.umass.edu/phil100-pgraham Meditations on First Philosophy - Meditation 1 Meditations A link to this is on the website. But you can access it directly at But earlymoderntexts.org earlymoderntexts.org Course Website: Course Reading for next time: Rene Descartes’s Reading Review From Last Time What are the four core areas of What philosophy? philosophy? What are some examples of questions What that you might ask in each of these areas? areas? Arguments What is an argument? An argument is a series of propositions An meant to establish another proposition meant The series of propositions are the The argument’s premises premises The proposition those premises are offered The in an attempt to establish is the argument’s conclusion conclusion Some Non-Philosophical Arguments 1. 2. 3. 1. 2. 3. If you don’t support President Obama’s If health care plan, then you don’t love America. America. You don’t support President Obama’s You health care plan. health Therefore, you don’t love America. If you support President Obama’s health If care plan, then you are a socialist. care You support President Obama’s health care You plan. plan. Therefore, you are a socialist. Validity In philosophy we are particularly interested in In a special class of arguments, namely valid ones. ones. In philosophy, “valid” has a very specific meaning. A valid argument is one for which it is valid impossible for all of its premises to be true and its conclusion false. and Another way of putting it: a valid argument’s Another premises entail its conclusion. entail (The truth of the premises of a valid argument (The guarantee the truth of its conclusion.) guarantee Soundness An argument is sound if and only if An sound (1) (1) (2) (2) iit is valid, and t its premises are actually true. Every sound argument has a true Every conclusion. conclusion. Why? What’s So Great About Valid Arguments? If an argument is valid then, to know If whether we should accept the conclusion we only need to determine one thing: we 1. Are its premises true? Why? Because if its valid and its premises are true Because then we know that its conclusion must be true. then And if it is true, clearly we should believe it. Our Focus: Valid Arguments In this class, when we construct arguments we will In aim to construct sound arguments (valid + true premises). premises). Also, the philosophers we will be discussing are also Also, aiming to construct sound arguments. aiming Though there are many different types of valid Though arguments, we will focus on a couple specific forms of valid arguments. Modus Ponens Some valid arguments are valid merely in Some virtue of their form. virtue Any argument with the following form is Any valid: valid: 1. 2. 3. P If P, then Q Therefore, Q This argument form is known as modus This ponens (MP). ponens Modus Ponens (II) Any argument with the following form is also in Any modus ponens form. modus 1. 2. 3. If P, then Q P Therefore, Q The order of the premises does not matter. Any argument in modus ponens form is valid. And Any so any argument in modus ponens form that has true premises is sound. true Modus Ponens (III) 1. 2. 3. 1. 2. 3. Five is a number greater than four. If five is a number greater than four, then five is a If number greater than two. number Therefore, five is a number greater than two. If five is a number greater than four, then five is a If number greater than two. number Five is a number greater than four. Therefore, five is a number greater than two. Modus Tollens Here is another valid argument form: 1. 2. 3. If P, then Q It is not the case that Q Therefore, it is not the case that P Therefore, This argument form is known as modus This tollens (MT). tollens Just as in the case of modus ponens, the Just order of the premises does not matter. order Any argument in modus tollens form with Any true premises is sound. true Modus Tollens (II) If Barack Obama lives in the If Whitehouse, then Barack Obama is President. President. 2. It is not the case that Barack Obama It is President. is 3. Therefore, it is not the case that Therefore, Barack Obama lives in the Whitehouse. Whitehouse. 1. Our Focus: MP and MT In this class we will try to construct In arguments in MP and MT form arguments The arguments of the philosophers we The will discuss will often be presentable in MP or MT form. MP Quiz: Validity and Soundness Can a valid argument have a false premise? SURE! Here’s an example: 1. 2. 3. If the moon is made of cheese, then the moon is If edible. edible. The moon is made of cheese. Therefore, the moon is edible. This argument is valid (it’s in MP form) but it This has a false premise. has Quiz: Validity and Soundness Can a sound argument have a false Can premise? premise? NO! By definition, if an argument is sound, By its premises are true. its Quiz: Validity and Soundness Can a valid argument have a false Can conclusion? conclusion? SURE! Here’s an example: 1. If the moon is made of cheese, then the moon is If edible. edible. 2. The moon is made of cheese. 3. Therefore, the moon is edible. This argument is valid (it’s in MP form) but it This has a false conclusion. has Quiz: Validity and Soundness Can a sound argument have a false Can conclusion? conclusion? NO! We proved earlier that every sound We What was that proof? argument must have a true conclusion. argument Learning to PEE Often in this class we will, and you will Often be asked to, PEE. be We will, and sometimes you will be We asked to: Present, Explain, and resent, xplain, Evaluate an argument. PEEing Story: Bert and Ernie If Bert is in the bar, then Bert has his If ID. ID. 2. It is not the case that Bert has his ID. 3. Therefore, it is not the case that Bert Therefore, is in the bar. is 1,2 MT 1. PEEing (II) Story: Preacher’s sermon 1. 2. 3. God says that eating shellfish is an God abomination. abomination. If God says that eating shellfish is an If abomination, then eating shellfish is an abomination. abomination. Therefore, eating shellfish is an Therefore, abomination. abomination. 1,2 MP ...
View Full Document

Ask a homework question - tutors are online