## Philosophical Terms

Certain terms crop up again and again in philosophical discourse with their meaning presupposed. This list is neither comprehensive nor the final word. See also:

### Common Philosophical Terms

Iff. Short for “if and only if”.

A fortiori (“Hence still more strongly”). All cats are fat, a fortiori Tibbles is fat.

A priori / a posteriori. A proposition is knowable a priori iff one could be justified in believing it on the basis of reason alone. If experience must in some way enter into the justification, it is said to be only knowable a posteriori.

Ad hoc (“To a specific end or purpose”). A claim is ad hoc iff it lacks independent motivation but is made instead solely so as to save the pet theory of the person making it.

Ad hominem fallacy (“To the person”). Criticizing a position by calling attention to irrelevant personal characteristics of someone who holds it.

Analytic / Synthetic. A sentence is analytically true iff it is true solely in virtue of the meaning of the expressions within it. If the meaning alone of a true sentence is not enough to ensure its truth, it is synthetically true.

Argument. A set of claims where (a) one of these claims is the argument’s conclusion, (b) the others are its premises, and (c) the premises are (rightly or wrongly) put forward as evidence for the conclusion.

Begging the question. An argument begs the question iff one of the premises of the argument covertly assumes the truth of the conclusion – that is, the argument is circular.

Converse. The converse of “If X, then Y” is “If Y, then X”. It’s contrapositive is “If not-Y, then not-X”. Only the contrapositive is logically equivalent to the original.

Deduction. A deduction is a valid argument.

Definiens. In a definition, the definiendum is the phrase being defined, the definiens is what defines the definiendum.

Empirical. Dependent on, or in some other way related to, experience.

Induction. An induction is an argument the truth of whose premises would not serve to guarantee the truth of its conclusion, yet would provide some evidence for it. Sometimes said to be “inductively but not deductively valid”. Two common types of inductive inference are…

Enumerative induction. To infer from the truth of many instances of a generalization, to the truth of the generalization itself, is to have performed an enumerative induction. E.g. this swan is white, this other swan is white, and so is that one, therefore all swans are white.

Inference to the best explanation. To infer to the best explanation is to infer from the existence of a phenomenon (e.g. tongue marks on the butter) to the truth of the theory that best explains the phenomenon (e.g. a mouse in the house).

Ipso facto (“By that very fact”). E.g. To be a person is ipso facto to have moral worth. (Similar: eo ipso.)

Metaphysics / Epistemology. Metaphysics has many specific branches but at the broadest level can be thought of as the study of how things are. By contrast, epistemology (again broadly) is the study of our knowledge of how things are.

Modus ponens. Any inference of the form: if X, then Y, X, therefore Y (“Affirmation of the antecedent”). Not to be confused with the fallacious: if X, then Y, Y, therefore X (“Affirmation of the consequent”).

Modus tollens. Any inference of the form: if X, then Y, not Y, therefore not X (“Denial of the consequent”). Not to be confused with the fallacious: if X, then Y, not X, therefore not Y (“Denial of the antecedent”).

Necessary / Contingent. A state of affairs is necessary iff it could not possibly have failed to obtain. It is contingent iff it obtains though it could have failed to obtain.

Necessary / Sufficient Condition. X is a necessary condition of Y iff Y could not obtain without X also obtaining. X is a sufficient condition for Y iff X’s obtaining is enough for Y to obtain.

Normative. A normative (or “prescriptive”) claim is one that could be true only if someone or other ought to do something, or something ought to be the case. A normative term is one that cannot be used except in making normative claims. Contrasted with (merely) descriptive claims/terms.

Ontology. A branch of metaphysics concerned specifically with what (kinds of) things there are.

Possible world. A way things could have been (or are, since the actual world is also a possible world).

Reductio ad absurdum (“Reduction to absurdity”). A good way to argue for a claim is to temporarily hypothesize the negation of this claim and then show that this hypothesis generates an absurdity.

Sound. An argument is sound iff (a) its premises are all true and (b) it is valid.

Straw position / Straw man. A position under criticism, but which no one really holds.

Thought experiment. An imagined scenario. Our intuitions about the scenario may be incompatible with what a theory claims about the scenario, forcing us to decide between the theory and our intuitions.

Type/Token. How many letters does the word “London” contain? The question is ambiguous. It contains 4 types of letter (d, l, n, o) but a total of 6 letter tokens.

Use/Mention. Mentioning a word involves talking about the word itself, not about what it refers to, which is what is done in using the word. E.g. London is smelly (use); “London” has six letters (mention).

Valid. An argument is valid iff the truth of all its premises would serve to guarantee the truth of its conclusion. (Alternative definition: ... iff there is no possible situation in which the premises are all true and the conclusion false.)

### Some Logical Notation

$\neg, \sim$: Not
$\wedge,\&$: And
$\vee$: Or
$\forall$: All/Every
$\exists$: There exists at least one/Some
$\rightarrow,\supset$: If...Then...
$[ ]$: Necessarily
$< >$: Possibly

$P\rightarrow Q$: If $P$ then $Q$
$[ ] Fa$: Necessarily, $a$ has property $F$
$\exists x (Fx\wedge Gx)$: There is at least one thing that is both $F$ and $G$
$\forall x(Fx\vee Gx)$: Everything is either $F$ or $G$