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Intro to Philosophy Unit 1: What Philosophy Is and Logic Basics

Philosophy is the careful study of basic questions about reality, knowledge, value and reasoning, and logic gives you the tools to judge whether an argument actually supports its conclusion. Every later unit depends on these tools.

Covers: Branches of philosophy, Arguments, premises and conclusions, Deduction and induction, Validity and soundness, Valid and invalid argument forms, Informal fallacies, Necessary and sufficient conditions

Study guide

Philosophy asks questions that cannot be settled just by looking something up: What is real? What can we know? How should we live? Its main branches divide these questions. Metaphysics studies the nature of reality, such as minds, causation, time and God. Epistemology studies knowledge and justified belief. Ethics studies right and wrong and the good life, and political philosophy applies similar questions to states and justice. Aesthetics studies beauty and art, and logic studies correct reasoning itself. What unites these fields is method: philosophers state positions clearly, give reasons for them, raise objections and replies, and test ideas against examples and counterexamples rather than relying on authority.

The basic unit of philosophical reasoning is the argument: one or more premises offered as reasons for a conclusion. Words like 'since,' 'because' and 'given that' often mark premises, while 'therefore,' 'so' and 'it follows that' often mark conclusions. Arguments come in two broad kinds. A deductive argument aims to guarantee its conclusion: if the premises are true, the conclusion cannot be false. An inductive argument aims only to make its conclusion probable, as when we generalize from a sample or predict the future from past patterns. Inductive arguments are called strong or weak, and they can be undermined by new evidence even when their premises stay true.

Two terms are tested constantly. A deductive argument is valid when it is impossible for all its premises to be true while its conclusion is false. Validity is a matter of form, not actual truth: 'All fish fly; salmon are fish; so salmon fly' is valid but has a false premise. An argument is sound when it is valid and all of its premises are true, so a sound argument always has a true conclusion. Some forms are always valid, like modus ponens (If P then Q; P; so Q) and modus tollens (If P then Q; not Q; so not P). Two look-alikes are invalid: affirming the consequent (If P then Q; Q; so P) and denying the antecedent (If P then Q; not P; so not Q).

Informal fallacies are errors in content or relevance rather than form. Ad hominem attacks the person instead of the argument. A straw man distorts a view to make it easy to defeat. A false dilemma presents two options when more exist. A slippery slope claims, without support, that one step will lead to disaster. Begging the question assumes the conclusion in the premises. Post hoc reasoning infers causation from mere sequence. Equivocation shifts the meaning of a word mid-argument, and composition and division wrongly transfer properties between parts and wholes. Finally, recall necessary and sufficient conditions: X is necessary for Y if Y cannot occur without X; X is sufficient for Y if X guarantees Y. 'If P then Q' says P is sufficient for Q and Q is necessary for P.

Key ideas

• Philosophy uses reasons, objections and counterexamples rather than authority to test answers to basic questions.
• An argument is a set of premises offered in support of a conclusion.
• Deductive arguments aim to guarantee their conclusions; inductive arguments aim to make them probable.
• Validity is about form: true premises would force a true conclusion.
• Soundness requires both validity and true premises, so sound arguments have true conclusions.
• Modus ponens and modus tollens are valid; affirming the consequent and denying the antecedent are not.
• Informal fallacies such as ad hominem, straw man and begging the question fail because of irrelevance, distortion or circularity.

Formulas and rules

Common mistakes

• Calling an argument invalid just because its conclusion is false, when the real problem is a false premise.
• Assuming an argument is valid because all of its statements happen to be true.
• Confusing affirming the consequent with modus ponens because both mention P and Q.
• Reversing necessary and sufficient conditions when translating 'if P then Q.'
• Labeling any personal criticism ad hominem, even when the person's credibility is actually relevant (such as a witness who was paid).

Before the test, make sure you can

• Identify the premises and conclusion of an argument written in ordinary language.
• Name the main branches of philosophy and the questions each asks.
• Distinguish deductive from inductive arguments.
• Explain the difference between validity and soundness and evaluate examples for each.
• Recognize modus ponens, modus tollens, affirming the consequent and denying the antecedent.
• Identify common informal fallacies in short passages.
• Translate 'if P then Q' into necessary and sufficient conditions without reversing them.

Words to know

Metaphysics

The branch of philosophy that studies the basic nature of reality, such as existence, minds, causation and time.

Epistemology

The branch of philosophy that studies knowledge, justification and the limits of what we can know.

Ethics

The branch of philosophy that studies right and wrong action, moral value and the good life.

Argument

One or more premises offered as reasons to accept a conclusion.

Premise

A statement given as a reason in an argument.

Deductive argument

An argument meant to guarantee its conclusion if its premises are true.

Inductive argument

An argument meant to make its conclusion probable, such as a generalization from a sample.

Valid

Said of a deductive argument whose form makes it impossible for the premises to be true and the conclusion false.

Sound

Said of a deductive argument that is valid and has all true premises.

Modus ponens

The valid form: If P then Q; P; therefore Q.

Modus tollens

The valid form: If P then Q; not Q; therefore not P.

Affirming the consequent

The invalid form: If P then Q; Q; therefore P.

Denying the antecedent

The invalid form: If P then Q; not P; therefore not Q.

Ad hominem

A fallacy that attacks the person making an argument instead of the argument itself.

Straw man

A fallacy that misrepresents an opponent's view so it is easier to attack.

Begging the question

A fallacy in which the premises assume the conclusion they are meant to prove; circular reasoning.

False dilemma

A fallacy that presents only two options when others are available.

Necessary condition

X is necessary for Y if Y cannot happen without X.

Sufficient condition

X is sufficient for Y if X alone guarantees Y.

Sample practice questions

30 more questions like these, from level 1 to 5, are in the practice set.