Philosophical Glossary

A Glossary of Some Common Terms in Philosophy

Philosophers use some words and phrases that are not in common usage. Sometimes they also use words in ways that other people don’t. These are some words or phrases with unfamiliar meanings that you may encounter in your readings.

Another glossary is Jim Pryor’s, which can be found here. I have drawn some terms from there (but all of the definitions are my own). If you have more terms that belong on this glossary, please contact me.

actuality, necessity, possibility, contingency, plausibility

If a claim, belief, or statement is actually true, then it is true in the real world. The claim "it is Tuesday" is actually true on Tuesdays, and it is actually false on any other day.

If a claim, belief, or statement is necessarily true, then it is true and it could not have been false. For instance, it's necessarily true that 2+2=4. Two plus two could never have equaled anything else. The same is true for things that are necessarily false: 2+2=5 isnecessarily false. It could not have been true.

If a claim, belief, or statement is possibly true or false, then whether or not it is actually true or false, itcould have been otherwise. The claim "I have two potatoes" is possibly false even when I do have two potatoes, because it'spossible that I could have had three potatoes or no potatoes. Another word for possibility is contingency.

If a claim, belief, or statement is plausibly true or false then it is safe to believe that it is true or false. This doesn't mean it is for sure true or false, or necessarily true or false, or actually true or false.

a fortiori

"And therefore, there is even stronger reason to think that…" So, for instance, I might write "I don’t like to ride bicycles, soa fortiori I really don’t like to ride bicycles in the rain."

a priori, a posteriori

If I can know something "a priori," I can know it without having to first experience it or otherwise investigate the world. For instance, I can know that 2+2=4 a priori, because I don't have to do any science experiments or go observe anything in order to check this.

If I can only know something "a posteriori" then I need to have an experience of it or otherwise investigate the world to learn it. For instance, I can only know that a tablespoon of peanut butter has 95 calories a posteriori, because I can't figure this out just by thinking about it.

ad hoc

A premise is ad hoc if I add it to my argument merely to avoid an objection, not because there are any other good reasons for adding that premise. So, if I argue that I can cook any food you can name, and then you ask me to cook apple pie, and I respond that I can cook any food you name except for apple pie, my response is ad hoc. If something is entirely ad hoc, philosophers tend to think it is not very convincing, because there is no reason to accept it.

affirming the consequent

A formal fallacy that takes the following form:

1. If it’s raining outside, then the ground is wet. (Premise)

2. The ground is wet. (Premise)

3. Therefore, it’s raining outside. (Conclusion, from premises 1 and 2)

This is fallacious because there are many other ways for the ground to be wet. Maybe someone spilled a bucket of water.


An argument is a series of premises and at least one conclusion. The premises provide support for the conclusion. Here is a very simple argument:

1. Socrates is a human being. (Premise)

2. All human beings are mortal. (Premise)

3. Therefore Socrates is mortal. (Conclusion, from premises 1 and 2)

If the premises necessitate or guarantee the conclusion (in other words, the conclusion must follow from the premises) we say that the argument is a deductive argument. If the premises make the conclusion more likely but don't guarantee the conclusion we say that the argument is an inductive argument.

Philosophers will often use "strong" and "weak" in an unusual way to describe arguments. If the argument is an argument that has a surprising conclusion, then it is a strong argument. If the argument has a very simple conclusion that is not very surprising, then it is a weak argument. (Notice that this is very different from how we normally talk: normally a strong argument is a convincing one, and a weak argument is an unconvincing one. In philosophy, strong arguments are often less convincing, because they are trying to establish something that is hard to establish, whereas weak arguments can be very convincing, because they are not saying anything that is hard to support.)

beg the question

In philosophy, "beg the question" means "assume the answer." It does not mean "raise the question." When writing philosophy, only use "beg the question" in the special philosophical sense.

ceteris paribus

"Everything else held equal." For example, I might say "if you take the left path rather than the right, then ceteris paribus you will get there five minutes faster." I might say that because if something else happens, you might end later or earlier.


When you see "cf. [Some source]," the author is making a reference to that source and suggesting that you can compare that source to what the author is saying. Often this is because the source is saying something different.


A conditional is a statement of the form "if... then..." For instance, "if you pet a cat, it will purr" is a conditional. The "if..." part of a conditional is known as the antecedent. The "then..." part is known as the consequent.

criterion, criteria

‘Criterion’ is the singular form of ‘criteria.’ A criterion is a standard that we use to make a judgment. For example, my criterion for deciding which sandwich to eat might be "which sandwich tastes best?"

de facto, de jure

"De facto" is about how the world is. "De jure" is about what the laws say. So for instance I might say "de jure the rule is that you have to come to a complete stop at a stop sign, but de facto many drivers only slow down a lot."


"For example."


Empirical claims are claims about things that we can observe in the world. Science is a process which is almost entirely involved in figuring out empirical things. Empirical questions are things like "how much does this potato weigh?" and "if we raise taxes, will revenue increase, and by how much?"


A fallacy is a possible mistake in reasoning. There are two kinds of fallacies: formal fallacies and informal fallacies. A formal fallacy is a mistake that is always a mistake. An informal fallacy is sometimes a mistake and sometimes not a mistake. Many lists of fallacies, both formal and informal, can be found online. It is typically not worth remembering their names.


"That is."

ipso facto

"For that reason." So for instance I might say "if you can draw a square then ipso facto you can draw a rectangle."

modus ponens and modus tollens

Modus ponens is a form of argument that goes like this:

1. If X, then Y.

2. X.

3. Therefore, Y.

For example:

1. If I was born in America, I am an American.

2. I was born in America.

3. Therefore, I am an American.

Modus tollens goes like this:

1. If X, then Y.

2. Not Y.

3. Therefore, not X.

For example:

1. If I was born in America, I am an American.

2. I am not an American.

3. Therefore, I was not born in America.

non sequitur

"It doesn’t follow." Typically philosophers will use this to say that the premises in an argument do not support the conclusion of the argument.

normative, positive

Normative statements are statements about what we should do or about which things would be better and worse. Here are some normative statements:

It is bad to abuse animals.

You shouldn't steal food from hungry people.

Positive statements are about how things are. Here are some positive statements:

Some people abuse animals.

Yesterday I stole some food from a hungry person.

Another way to put it is that normative statements "judge" or "evaluate" the world and things in it, whereas positive statements just describe the world and the things in it without saying whether those things are good or bad.

Sometimes non-philosophers will use the word ‘normative’ to mean "normal" or something similar. This isn't even really correct outside of philosophy. Non-philosophers will also sometimes use ‘normative’ to describe something that relates to some sort of rule or standard (that is, some sort of norm). In philosophy, only use ‘normative’ to describe judgments.


When philosophers use the word ‘obtain’ they usually use it as a verb which means "is the case." So if I say "the state of affairs in which all peanut butter sandwiches are six feet wide does not presently obtain," I mean that all peanut butter sandwiches are not six feet wide.


"Despite what this person argues." So for instance I might say "pace your claim that I can’t eat four sandwiches in five minutes, I’ve just done precisely that."

per se

"In itself."

prima facie

"On its face" or "at first glance."

pro tanto

A pro tanto reason is a reason to do something that can be outweighed by other reasons. The fact that a sandwich is tasty is a pro tanto reason to eat it. There might be other reasons not to eat the sandwich - maybe it’s not yours. Still, you do have a pro tanto reason to eat it.


In philosophy, ‘prove’ is a success term (see below for an explanation of that phrase).

red herring

A red herring is a detail that seems important but that actually is not important.

reduce, reduction

If X can be reduced to Y, this means that X can be explained just in terms of Y without having to add anything else.

So, for instance, if we can reduce chemistry to physics, then this means we can explain chemical reactions just by giving explanations in terms of physics (atoms and so on) without having to add any special chemistry terms (like acid or base). If something is irreducible this means there is something special about it that can't be explained in terms of something else.

reductio ad absurdum

A reductio ad absurdum argument (also known as a reductio) is an argument that shows how ridiculous, absurd, or clearly false conclusions follow from certain premises. If the reductio argument is convincing, then because the conclusions are so absurd, one or more of the premises must be false. Here is a reductio to show that you cannot go back in time to kill your parents:

1. You can go back in time to kill your parents.

2. If you go back in time and kill your parents, then you will never be born.

3. If you will never be born, then you could never have gone back in time and killed your parents.

4. Therefore, if you go back in time and kill your parents, then you couldn't have gone back in time and killed your parents.

5. But that makes no sense. (It is absurd.) We must have gone wrong somewhere. Premise #1 seems to be the only part of the argument that we could doubt. So premise #1 must be false.

Notice that in order to make my reductio argument I had to first assume to be true the thing I am trying to disprove. (I had to assume that you can go back in time to kill your parents.) This is because the argument works by showing that, if we take a premise to be true, it gives us absurd conclusions, so we have to reject the premise.


When you refute an argument, you show why it is false. This means ‘refute’ is a success term in philosophy.


"Unconditionally" or "without any qualifications." So, I might say "some sandwiches are good for satisfying hunger, some are good sources of nutrition, and some taste great, but are any sandwiches good simpliciter?"

straw man

A straw man argument is an implausible argument that is created and then attributed to one's opponent. It is not good to make a straw man argument because it is important to argue against an opponent's strongest arguments, not against made-up, weaker arguments.

success term

A success term is a word or phrase that implies that we’ve gotten something right. For instance, in philosophy, ‘refute’ and ‘prove’ are both success terms. If I say you’ve refuted something or you’ve proven something, this means that you’ve succeeded in refuting or proving what you set out to refute or prove. If you say "Sandel refutes Rawls," this means Sandel is right. You can write "Sandel attempts to refute Rawls" if you don’t want to imply Sandel is right.

sui generis

"Unique" or "one of a kind."


Philosophers often use ‘testimony’ to refer to information that you get from someone else, rather than getting that information directly yourself. If I tell you that there are four rocks on the ground, this is testimony.

true/truth, false/falsity, valid/validity, sound/soundness

A valid argument is an argument which has a conclusion that follows from its premises, assuming the premises are true.

A valid argument with true premises is also a sound argument. A valid argument with one or more false premises is unsound.

Premises and conclusions are true or false. The premise or the conclusion "cats have seventeen legs" is false.

You shouldn’t call a premise or a conclusion "valid" or "invalid" or "sound" or "unsound." Those are terms that apply to arguments. You shouldn’t call an argument "true" or "false." Those are terms that apply to premises and conclusions.


"In other words."