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These two seem quite interchangeable—is it so or is it just me unable to comprehend the difference?

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    Don't forget prima facie which also has a similar but subtly different meaning in legal contexts.
    – dodgethesteamroller
    Commented Sep 14, 2015 at 7:58
  • @dodgethesteamroller Prima facie - 'at first appearance' - slightly different to 'the fact alone' - I think. There is prima facie evidence of murder, in the form of a smoking gun; but possession of a smoking gun is not ipso facto evidence of guilt
    – WS2
    Commented Sep 14, 2015 at 8:46
  • Technically, this belongs here then : ))
    – moonwave99
    Commented Sep 14, 2015 at 11:46
  • @moonwave99 Does it? I think the thrust of the OP's question is about the usage of per se and ipso facto in English, primarily in academic and legal English, and I threw prima facie into the mix as another possible candidate with a similar meaning. I think the discussion involves Latin only incidentally; the important thing is how the two (or three) terms are used in English, even if borrowed unchanged from the Latin. (It's kind of analogous to arguing about whether chaise lounge is incorrect—that wouldn't belong in a hypothetical French SE but here.)
    – dodgethesteamroller
    Commented Sep 14, 2015 at 14:28
  • @WS2 Great example of prima facie vs ipso facto in legal English. I'd like to see this expanded into a separate question and answer.
    – dodgethesteamroller
    Commented Sep 14, 2015 at 14:30

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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.

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    The meaning "in itself" is a reasonable equivalent English phrase, but it doesn't express the wide range of meaning of "per" in Latin very clearly. The relevant meanings of "per" in this context express means - "through / by / by means of / with / by way of itself" or causation - "because of / on account of itself". More explanatory translations of "ipso facto" (which is in the Ablative case) would be "by means of the fact itself" or "because of the fact itself".
    – alephzero
    Commented Sep 14, 2015 at 1:11
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    @D4RKSOUL "Ipso facto" can only be applied to a fact (ipsum factum = the fact itself). "Per se" can by applied to any thing (se = itself) . A fact can be considered to be a thing. But in "Possession of a vehicle's registration document is not per se evidence of ownership" you would have to stop to decide if the thing that "per se" referred to is "the registration document" (wrong) or "Possession of the registration document" (right).
    – alephzero
    Commented Sep 14, 2015 at 1:54
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    It might be worth adding that 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.
    – alephzero
    Commented Sep 14, 2015 at 11:56
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    @D4RKS0UL It's not "wrong", but (as I tried to explain) I think it is slightly ambiguous what the "per se" refers to. "The registration document per se does not name the owner of a vehicle" would be good, and unambiguous. (The registration document actually names the "registered keeper" of the vehicle, i.e., the person responsible for ensuring that it is used legally. It may be owned by somebody else, for example by the finance company which loaned you the money to buy it.)
    – alephzero
    Commented Sep 14, 2015 at 12:07
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    @D4RKS0UL Adverbs and particularly adverb phrases can modify almost anything, not just verbs. See dictionary.cambridge.org/grammar/british-grammar/adverb-phrases. They usually occur immediately before or after the words they modify. The commonest way to parse a sentence is to assume they modify the smallest phrase that makes sense, so in "... the r.d. per se ....", the "per se" modifies "r.d.". On the other hand in "... the r.d. ipso facto ...", ipso facto can only refer to a fact. "The r.d." is a thing, not a fact, so ipso facto must refer to "Possession of the r.d."
    – alephzero
    Commented Sep 14, 2015 at 19:19
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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.)
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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.)

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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.

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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..."

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