These two seem quite interchangeable—is it so or is it just me unable to comprehend the difference?
The meanings are close, but I think the main difference is the context in which each is used.
Per se, meaning 'in itself' is used in sentences such as this, from the OED:
1992 New Republic 13 July 4/3 Real conservatives do not reject homosexuality per se (in itself) so much as they reject victimology.
Ipso facto meaning 'the fact itself', or 'the fact alone' tends to be used by lawyers in statements such as:
Possession of a vehicle's registration document is not ipso facto (by that fact alone) evidence of ownership.
The word fact looks like the obvious English translation of facto, but it doesn't quite capture the meaning of the Latin. The root is the verb facere which means to do or to make. So factum literally means a/the thing that is/was done/made. The English word "fact" also includes more abstract ideas. "All triangles have three sides" is a fact in English, but not really a factum in Latin.
Ipso facto is used to present one fact as a reason for another, whereas per se is used to narrow a noun.
To expand upon the examples that WS2 offers:
- ipso facto relates vehicle registration to vehicle ownership.
- per se narrows homosexuality to exclude victimology. (Obviously I'm just using the example as given here.)
The term per se means the thing itself, to the exclusion of anything implied, derived, construed, or represented. The very thing named.
"It's not that I dislike cats per se. But I am allergic to them."
Lacking context, I would label this term "philosophical" in nature. (The nature of a thing.)
The term ipso facto implies a causality, one thing being named having another thing as a necessary consequence due to the first thing's existence.
"The goods were found in his possession, ipso facto he was involved in some way."
(Bad example but I cannot come up with a better one right now.)
Lacking context, I would label this term "judicial" in nature. (The meaning / consequence of a thing.)
People have said variations of this, but no one seems to have said clearly:
Per se refers to things, whereas ipso facto refers to facts. Things exist (or don't); facts are true or false.
Look at all the examples provided:
- Possession of a smoking gun is not ipso facto evidence of guilt.
- Possession is a fact: it is either true or false that the defendant possesses a smoking gun.
- Real conservatives do not reject homosexuality per se (in itself) so much as they reject victimology.
- Homosexuality is a thing: it either exists or it doesn't.
- That the delegates could come to an agreement indicates ipso facto that they acknowledged...
- Whether the delegates could come to an agreement is a fact: they either could or could not come to an agreement.
- It's not that I dislike cats per se. But I am allergic to them.
- Cats are a thing: they either exist or don't.
I had to read this thread a few times over to get it.
Ipso facto is less common in my experience and always has the meaning of 'in and of itself, without extraneous factors'. "That the delegates could come to an agreement indicates ipso facto that they acknowledged..."
Per se can also have this meaning, which is what it originally meant in Latin, but it is commonly used now to mean 'as expected, exactly, technically, as such'. "The witness didn't answer the question per se, but he did imply..."