Philosophical Terms and Methods
A Philosophical Glossary for Beginners


Argument structures that are always bad


See discussion here.

begging the question

See discussion here.

Argument structures that can (sometimes or always) be good

reductio ad absurdum

See discussion here.

shifting the burden of proof

See discussion here.


A dilemma is a form of reasoning that presents a choice between two alternatives. Here is an example: If P then Q. And if R then also Q. But either P or R. So in any event, Q. Dilemmas are perfectly respectable forms of argument.

In an argument of this sort, P and R are called “the horns” of the dilemma. If you want to reject a dilemma, then you have several choices:

A dilemma where the options do not exhaust the relevant possibilities is called a false dilemma. Here’s an example:

Caliph Omar ordered the destruction of the Library at Alexandria, proclaiming that the books either contained the same doctrines as the Koran, and so were unnecessary, or else they contradicted the Koran, and so were pernicious. In either case, they should be destroyed.

This isn’t a good argument that the books should be destroyed, because it hasn’t considered all the possibilities: what if the books at Alexandria talk about different things than the Koran, and so neither contain the same doctrines nor contradict the Koran?

Here’s another example of a false dilemma. Can you explain why?

Should the government take total control of the software industry, or must we allow companies like Google to be completely free of government regulation?

arguments by analogy

Sometimes analogies between two situation give reason to believe that some facts known to be true in the first situation also apply in the second.

But arguments from analogy are tricky, because two situations will always differ in some respects, and such arguments rely on those differences not mattering for the issue under discussion.

An opponent may protest that the difference do matter — and then back that complaint up with an explanation of how and why they matter (or some other reasons for believing that they matter, even if we don’t yet know how and why).

slippery slopes

These are arguments of the form: If we accept A, then for the same reasons we’ll also have to accept B, and then C, and then…, where the consequences get worse and worse. This could be a matter of how morally/practically bad they are, or it could be a matter of how implausible they are.

These sorts of arguments aren’t necessarily bad. If it really is true that the same reasons alleged to support A would then also support further things we’re sure are unacceptable, then perhaps those reasons don’t really support A, in the first place.

But often these sorts of arguments exaggerate how inevitable it would be to continue down the “slippery slope.” There may be good reasons why although some of the starting steps are justified, the later, less acceptable ones are not.

ad hominem arguments

An ad hominem argument is an argument that attacks a claim on the basis of features of the person who holds it. Two different kinds of argument are called “ad hominem arguments.” One of these kinds are fallacious arguments; the others are perfectly respectable.

The fallacious version is where you criticize someone’s argument or proposal because of logically irrelevant personal defects. For instance:

His claims about relationships must be false because he's a philanderer.


His political arguments must be bad because he doesn't know what he's talking about.

If someone offering an argument or proposal has a bad character, or may be hypocritical or have unsavory motives, these may be good reasons to question things they’re saying that we don’t independently know to be true. (We may reasonably decline to take their word for it.) But sometimes we are already in a position to evaluate their argument or proposal, without needing to trust or rely on the integrity of its author. And in these cases we should remember that scoundrels and fools — even if they are insincere or unjustified in their beliefs — might nonetheless turn out to be right. And authorities no matter how eminent can be wrong. The source of a view is one thing, and whether there are good reasons to hold the view is something else.

The respectable argument called an “ad hominem argument” consists in objecting to someone’s claim on the grounds that it’s incompatible with other views she holds — regardless of whether you regard those other views as correct.

For instance, suppose Maxine says:

The U.S. Postal Service is very unreliable. I think we should allow private, for-profit companies like FedEx and UPS to compete on an equal footing with the Postal Service.

Then Sally objects:

But Maxine, you are a communist!

Sally is not just calling Maxine a name. Sally’s point is that Maxine’s previous commitments force her to support state control and oppose private enterprise, and these commitments conflict with the view she’s advocating now. This is a perfectly legitimate criticism of Maxine.

Philosophers generally use the phrase “ad hominem argument” in the second sense.

More philosophical vocabulary

ad hoc

You call something ad hoc when it’s introduced for a particular purpose, instead of for some general, antecedently motivated reason. So, for instance, an ad hoc decision is a decision you make when there’s no general rule or precedent telling you what to do.

Philosophers sometimes accuse their opponents of making ad hoc hypotheses (or ad hoc stipulations, or ad hoc amendments to their analyses, etc.). These are hypotheses (or stipulations or amendments) adopted purely for the purpose of saving a theory from difficulty or refutation, without any independent motivation or rationale. They will usually strike the reader as artificial or “cheating.”

For instance, suppose you analyze “bird” as any creature that can fly. I then cite mosquitos as a counter-example. They can fly, but they aren’t birds. Now, you might fix up your analysis as follows:

A bird is any creature that can fly, and which is not a mosquito.

This would be an ad hoc response to my counter-example. Alternatively, you might fix up your analysis as follows:

A bird is any creature that can fly, and which has a backbone.

This would be an independently motivated, and more appropriate, response to my counter-example. (Of course, someone may present counter-examples even to this revised analysis.)


In a philosophical discussion, calling an expression or form of speech “ambiguous” means it has more than one acceptable meaning. See discussion here.

Don’t call an expression “ambiguous” just because different people have different views or theories about it. Different people have different views about what it means to be good, but that wouldn’t yet show that “good” is ambiguous. It would just show that there’s some controversy over what “good” means.

Neither should you call an expression “ambiguous” just because it’s vague, or imprecise, or difficult to know what the correct philosophical theory of it is.

When an argument illegitimately trades on an ambiguity, we say that the argument equivocates.

consistency, compatibility, contradiction

When a set of propositions cannot all be simultaneously true, we say that the propositions are inconsistent. Here is an example of two inconsistent propositions:

  1. Oswald acted alone when he shot Kennedy.
  2. Oswald did not act alone when he shot Kennedy.

When a set of propositions is not inconsistent, then they’re consistent. Note that consistency is no guarantee of truth. It’s possible for a set of propositions to be consistent, and yet for some or all of them to be false.

Sometimes we say that a proposition P is incompatible with another proposition Q. This is just another way of saying that the two propositions are inconsistent with each other.

A contradiction is a proposition that’s inconsistent with itself, like P and not-P.

Sometimes it’s tricky to see that a set of propositions is inconsistent, or to determine which of them you ought to give up. For instance, the following three propositions all seem somewhat plausible, yet they cannot all three be true, for they’re inconsistent with each other:

  1. If a person promises to do something, then they’re obliged to do it.
  2. No one is obliged to do things which it’s impossible for them to do.
  3. People sometimes promise to do things it’s impossible for them to do.


Philosopher Smith is equivocal here means that he gives some argument which equivocates. It does not mean that he’s neutral or agnostic about the matter. Nor does it mean he can’t make up his mind. (These might be explanations of why he equivocates; but you shouldn’t use the phrase He equivocates to describe his neutrality or agnosticism or indecision.)

falsehood and fallacy

A fallacy is an error in one’s inferences or argument. A falsehood is an error in the claims one makes.

Claims, beliefs, and statements are true or false. Only inferences and arguments can be fallacious.

imagine, conceive

To imagine or conceive of some possibility is to form an idea of it, to entertain that possibility in your mind. When you imagine some possibility, you are not committing yourself to the claim that that possibility actually obtains or is likely to obtain.

infer and imply

Inferring is the psychological activity of drawing conclusions from premises. Only people can infer. So don’t say:

This argument infers that…

What the argument does is imply or entail a conclusion. It doesn’t infer it.

In addition to arguments implying things, sometimes we talk about people implying things. In this usage, implying is an activity, but it’s a different activity than inferring. For instance:

Sarah implied that I was a fool.

means that Sarah suggested that I was a fool, without explicitly saying so.

But in the primary usage of these words, implying is something premises and arguments do: they imply their conclusions. And inferring is something people do. People infer by looking at the evidence and deciding what hypothesis that evidence best supports.


In everyday speech, people often use the word “logical” like this: John’s attitude to smoking just isn’t logical, or: Spock is incapable of emotion because he tries to be so logical.

You should not speak this way in philosophical discussions. In philosophy, the word “logic” has a special technical meaning. (If you want to know what it is, you’ll have to take some courses in logic.) You should say instead:

John’s attitude to smoking is unreasonable.

Also, don’t say such things as: That was a logical point, or That was a logical objection, or This is a logical argument.

Say instead:

That was a fair or convincing point.


That is a reasonable objection.


This is a valid or persuasive argument.


Something is meaningless if it is nonsense, like “XH$%^IE”. Don’t say that a claim is meaningless if all you mean is that it is false.

proposition, concept

A proposition is something that you could hold, or believe, or put forward as a claim. It’s capable of being true or false. It’s expressed in language by a complete sentence.

A concept is usually expressed in language by a noun phrase, not by a sentence.

So, we have “the concept of electricity,” and “The proposition that Socrates was a philosopher.”

refuting and proving

Refuting a claim is showing it to be false — typically by producing reasons that make it clear that it’s false. Until you produce reasons, you may deny or reject the claim, but you won’t have refuted it.

In addition, don’t say:

Berkeley refutes Locke’s claim that there are material objects.

unless you think that Berkeley has succeeded in demonstrating that Locke’s claim is false. If Berkeley has refuted Locke, then Locke must be wrong. You can’t write: Berkeley refuted Locke’s claim, but in fact Locke was right.

If you doubt or want to leave it open whether Berkeley’s criticisms of Locke are successful, you should say instead:

Berkeley denies Locke’s claim that…


Berkeley argues against Locke’s claim that…


Berkeley rejects Locke’s claim that…


Berkeley tries to refute Locke’s claim that…

Similarly, you should not say that Locke has proven some claim, or shown or demonstrated that something is the case, unless you think that Locke’s arguments for his claim are successful. If Locke has proven a claim, then the claim must be true.

If you doubt or want to leave it open whether Locke’s arguments for a claim are successful, then you should say instead:

Locke argues that…


Locke defends the claim that…


Locke tries to prove that…

or something of that sort.

thought and things

The Mississippi River and my idea of the Mississippi River are two very different things. One of them (the river) has existed since before I was born. The other (my idea of the river) has only existed since I first heard about the Mississippi River.

Nevertheless people often confuse thoughts with things. Don’t write like this:

Descartes realizes that even if all things are false, still he is thinking about those things, and if he is thinking about them he must exist.

You should instead say something like this:

Descartes realizes that even if all his thoughts or beliefs are false, thinking falsely is still a form of thinking, and if he is thinking at all then he must exist.


Philosophers call a term “vague” when there’s no sharp borderline between cases where the term applies and cases where it doesn’t apply.

So, for instance, it’s a vague matter how few hairs on your head makes you bald, or how many dollars in your bank account makes you rich, or how many grains of sand it takes to make a heap.

“Vague” does not mean “ambiguous.” Nor does it mean “unclear” or “difficult to understand.” Consider the following sentence:

The point of this essay is to prove that human beings never perceive material objects themselves, but only the a priori interface between a phenomenal object and its conceptual content.

This doesn’t mean anything. It’s just a bunch of words I put together in a way that doesn’t make any clear sense. You can call such prose “opaque,” or “difficult to understand,” or “gibberish.” Don’t call it “vague.”

valid, sound, true

In philosophical discussions, usually only inferences or arguments can be valid or sound. Not points, objections, beliefs, questions, or worries.

(Logicians do sometimes talk about sentences being valid, but this is a technical label that doesn’t mean what you probably think. It’s connected to the more usual philosopher’s talk of arguments being valid.)

For points and beliefs and statements, what you probably want to say is that they’re true (or false). Or that they’re justified or well-supported (or undefended or controversial).

In our courses, don’t call a statement “valid” or “sound.” Don’t call an inference or an argument “true.”

More foreign phrases used in philosophy

You may come across some of these in the readings. Most of these are from Latin.


“for example”


“that is”




“compare,” “see”

a fortiori

“even more so,” or “all the more so,” as in:

If all donkeys bray incessantly, then a fortiori all young donkeys bray incessantly.

ceteris paribus

“other things being equal,” or “other things happening normally,” as in the following dialogue:

Henry: Careful! You almost dropped the vase. If you dropped it, it would shatter, and Mom would kill us.

Lola: It might not have shattered. Maybe a gust of wind would have blown the pillow off the couch just as I dropped it, and it would have landed on the pillow.

Henry: You know what I mean. If you had dropped the vase, then, if things had otherwise happened normally, the vase would have hit the ground and shattered.

de facto, de jure

de facto means “in fact,” or “as a matter of fact”; de jure means “as a matter of law.” Examples:

In this town, the clergy have de facto immunity to the traffic laws. In the eyes of the law, of course, a speeder is a speeder; but no cop hereabouts would actually give a clergyman a speeding ticket.

The old brigand wielded a de facto authority over his pack of thieves — though of course he had no legal authority.


“substitute, imitation”

ipso facto

“by that very fact,” as in:

Anyone who wears chartreuse socks is ipso facto unfit to make fashion decisions.

non sequitur

“it doesn’t follow.” The premises do not support the conclusion.


“despite what X says,” as in:

Pace Freud, it is unusual for young boys to form sexual attachments to their mothers.

per se

“itself,” as in:

It’s not leisure per se which turns the mind to criminal pursuits; but rather the boredom which usually accompanies leisure.

prima facie

“at first glance,” as in:

Prima facie, it seems that George will inherit control of most of father’s estate; but the will is complicated, and our lawyers are looking into it even as we speak. Perhaps they’ll discover some clause that blocks George’s inheritance.


“without qualification,” as in:

There are good leaders, good businessmen, and good fathers. But is there to be found anywhere in the world a man who is good simpliciter?

sui generis

“unique, one of a kind,” as in:

Greed is an appetite, like hunger and sexual desire. It’s not the same thing as hunger or sexual desire, though. It’s akin to them, but in other respects it’s different. So there are more than just those two sorts of appetite. Greed is a sui generis appetite.