A Brief Introduction to Philosophy

What is philosophy?

A Brief Introduction to Philosophy

Philosophia - "love of wisdom"

• Popular conception: philosophy is the study of “deep things” – the meaning of life, human existence, society, art, etc.
• My definition: philosophy is the study of arguments – critiquing, producing, and analyzing them

Philosophical Arguments

• An argument is a series of sentences that are intended to provide reasons for accepting a conclusion.

Philosophical Arguments

• How do you convince someone to believe something that they don't believe?
• Start with some things that they do believe, and then show them that if they accept those things, they are (rationally) compelled to accept the conclusion.

Philosophical Arguments

PREMISES

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CONCLUSIONS

Philosophical Arguments

• Premises - The statements that form the basis of the argument (raw material). The opponent is supposed to already accept these statements.
• Conclusion - The thing you are trying to prove/establish/defend.
• To get from the premises to the conclusion, you connect the premises by drawing out their logical consequences.

Philosophical Arguments

EXAMPLE

(Premise 1) Jane is taller than Bob.

(Premise 2) Bob is taller than Dave.

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(Conclusion) Jane is taller than Dave.

Philosophical Arguments

We say that the premises imply or entail the conclusion.

Or, the conclusion is a consequence of the premises.

Validity & Soundness

• Validity - An argument is valid just in case the premises really imply the conclusion.
  I.e., if the premises are true, then the conclusion must be true.

• Validity does not require that the premises or the conclusion be true.

• Soundness - An argument is sound just in case it is valid and the premises are true.

Validity & Soundness

Example of a valid argument

(P1) Snow is blue.

(P2) The ocean is blue.

(C) Therefore, snow is the same color as the ocean.

Validity & Soundness

Example of a valid argument

(P1) Either today is Saturday or 2+2=5.

(P2) Today is not Saturday.

(C) Therefore, 2+2=5.

Validity & Soundness

Example of an invalid argument

(P1) If it's raining today, then I should bring an umbrella.

(P2) It is raining today.

(C) Therefore, cows drink milk.

Validity & Soundness

VALID or INVALID?

(P1) If John walks to work, he will be late.

(P2) John does not walk to work.

(C) Therefore, John will not be late.

Validity & Soundness

VALID or INVALID?

(P1) Most basketball players are tall.

(P2) LeBron James is a basketball player.

(C) Therefore, LeBron James is tall.

Paradoxes

A paradox is an argument that appears convincing, but the conclusion is absurd.

Paradoxes

The Paradox of the Heap

1. One grain of sand is not a heap of sand.
2. If a given amount of sand is not a heap, adding one grain of sand will make it a heap.
3. So, two grains of sand are not a heap.
4. By the same reasoning, three grains of sand are not a heap.
5. By the same reasoning, four grains of sand...
6. So, by starting with one grain of sand, and adding to it a grain at a time, we will never have a heap.
7. Therefore, there are no heaps of sand.

WTF, mate?!

Paradoxes

The Paradox of the Heap

1. Plucking one hair from a person who is not bald will not make that person bald. (A person is already bald before the last hair is plucked.)
2. So, if we start with a person with a full head of hair, and pluck his/her hairs one at a time, we will never make that person bald.
3. Therefore, there is no such thing as baldness.

Paradoxes

The Paradox of the Heap

1. It is wrong to have an abortion on the last day of a normal pregnancy.
2. But, surely, if it is wrong to have an abortion on a given day, it is equally wrong to have it the day before.
3. So, if it is wrong to have an abortion on the last day of a pregnancy, it is wrong to have one on the second-to-last day (and the third-to-last day)… etc.
4. But, then, it is equally wrong to have an abortion at every day in the pregnancy, and so, by our opening assumption, always wrong.

Paradoxes

The Paradox of the Heap

1. Humans are said to have evolved from apes. (Not quite true, but suppose)
2. So, every human has some ape as an ancestor.
3. If some animal is an ape, then every offspring of that animal is an ape as well.
4. So, it is impossible for an ape to have a non-ape (or a human) as an offspring.
5. So, it is impossible for anything that is not an ape to be a descendant of an ape.
6. So, it is impossible for any human to have an ape as an ancestor.
7. Therefore, humans did not evolve from apes.