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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
It is also a bit "too basic" for the site at this point -- @Thr4wn please let me know if you would like me to push this to ELU for you – Joseph Weissman Sep 23 '11 at 17:05
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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
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2 Answers

up vote 2 down vote accepted

Yes. They are known as tautologies.

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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
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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
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@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

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