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Consider the following propositions:

"I am me"
"I am my Father's son"

In both these cases, the predicate is the same as the subject by definition of the very subject and predicate.

Is there a special name for these kinds of propositions?

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Shouldn't this be in English.SE? – stoicfury Sep 23 '11 at 3:31
This does seem to fall fairly neatly under EL&U -- and if it's as casual as OP indicates it doesn't really belong here. @Thr4wn, if it's alright with you I would like to migrate this over to English.SE? – Joseph Weissman Sep 23 '11 at 4:10
It seems to be technical usage that is specific to philosophy or mathematical logic. It might be answered well there at ELU, but just as likely here. – Mitch Sep 23 '11 at 15:18
I would rather let Joseph (or other admins) use this question as a precedent of what questions are allowed on this site. However, pending admin approval, this is what I would argue: that making a question about philosophical terminology is an intrinsically sufficient justification for a question on this site. In addition, because my intent is intentionally constrained to knowing terminology as used in philosophy the question should not be moved to EL&U. Maybe close this question by some grounds, but knowing specifically how philosophers use terminology is outside the domain of EL&U. – Alexander Bird Sep 24 '11 at 5:38
@Thr4wn this would definitely be constructive/on topic here if you explicitly were asking for the meaning within a theoretical context -- but since you say you are just curious and don't have any philosophical context I'm not exactly sure where the 'philosophy' part of the question comes in. Please feel free to ping me in chat if you would like to discuss further. – Joseph Weissman Sep 24 '11 at 14:40
up vote 4 down vote accepted

In technical logic, a predicate is an entire statement. In your usage, with respect to grammar, the predicate is the verb and object (or other parts) which apply to the subject; the predicate is usually some relation about the subject. I think the latter concept of 'predicate' is what you are referring to.

In your statements, the subject is 'I' and the predicate is 'am me' or ' am my father's son'. The predicate relation, in both instances, is an equivalence relation ('is', equals'). The object of that particular predicate is proposed as equivalent to the subject. As such a proposition is then referred to as an equivalence.

The first one, because the pronouns refer to the same thing, is a tautology, because x=x is already an axiom of equivalence relations. The second is not, because you could be female.

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The OP's second example may be misleading, due the the quibble you suggest, but the answer to the actual question "Is there a special name for these kinds of propositions where the predicate is the same as the subject by definition?" is clearly tautology. – Michael Dorfman Sep 23 '11 at 16:30
I meant to use "proposition" in an Aristotle manner. This seems like an accepted technical usage of the word "logical proposition" and I correctly identified the subject and predicate according to the rules given on the wikipedia page I linked to. However, perhaps I needed to specify I was using that usage of the word proposition instead of assuming that people would know that. – Alexander Bird Sep 24 '11 at 5:30
@Michael: the OPs examples certainly are tautologies, but there are other tautologies not of this form. I was trying to give the more accurate answer. – Mitch Sep 24 '11 at 12:55
@Mitch: I agree that your answer was quite good, and very clear, and better fleshed out than mine. I just wanted to point out that, examples aside, he asked a direct question about propositions where the subject and predicate were true by definition, and it seems to me that the answer to that is more than a simple equivalence, but necessarily a tautology. (As an aside: I'm quite surprised you got downvotes for your answer.) – Michael Dorfman Sep 24 '11 at 14:12

Yes. They are known as tautologies.

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No i don't think there is a name. But as it stands you have NOT given an example of a sentence where the predicate IS the subject:

Your example: 'I am me'

Subject: 'I'.

Predicate: 'am me'.

The subject is not the same as the predicate because the subject contains one letter and the predicate four. Remember from grammar that 'subject' and 'predicate' are syntactic categories!

If you want a genuine example of a sentence who's subject is the same as the predicate then try these: a man a man, the dangerous dog the dangerous dog. is purple on the outside is purple on the outside...etc.. You just get ungrammatical nonsense.

Here is a sentence who's subject refers to the same thing as the predicate:

'The property of being red is red'.

Subject: 'The property of being red'.

Predicate: 'is red'.

Both of these refer to the property of being red. But like i said, i doubt these kinds of sentences have their own name.

Note also that identity sentences (the one's you use in your example) are not even ones where the subject merely refers to the same thing as the predicate:


S: 'a'.

P: '=a'.

The S refers to the thing a whereas the P refers to the property of being identical to a. These are not the same things.

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