It can be difficult to prove that a statement is meaningless because such statements are often offered in bad faith.

There are many classes of meaningless statement:

• A statement in argumentation may be considered meaningless because it draws a distinction without a difference, that is, asserts that two categories are disjoint without proposing a way to distinguish among them. For instance, the claim, "Pornography is different from erotica, but not in any particular way I can explain," may be considered to draw a distinction that makes no difference.
• A purported statement may be meaningless if its terms are undefined, or (more precisely) if it contains unbound variables. For instance, the sentence "All of the fronnicks have three sazzles apiece" is meaningless if the terms fronnick and sazzle are undefined (or unbound).
• A grammatically correct sentence may be meaningless if it ascribes properties to particulars which admit of no such class of properties. For instance, in the famous sentence "Colorless green ideas sleep furiously," the property of green color is ascribed to ideas, which do not have color.
• An ungrammatical sentence admits of no meaning. For instance, the string of words "deities Olympus Greek reside The. upon" forms no sentence, and cannot be said to have meaning as such.
• Adherents to various forms of positivism regard a statement as meaningless if it cannot be verified or falsified. For instance, Thomists make statements about indetectable essences of things, which by definition can never be sensed or perceived; positivists regard such statements as without meaning.