mi volas, ke li nun estu jam tute (mia filo), tio estas mia leĝa filo, kaj ke li estu nomata tiel, kiel mi, Dobĉinskij.

Surely the 'tio estas' in this quote is the equivalent of 'i.e.' (explanatory re-definition).

But let's consider some examples of where 'tio' is apparently used to refer to a person and ask what the tio represents.

I am called to the morgue to identify a body. I say to the police 'Tio estas mia patro.' Tio stands for the corpse (that thing), the body to be identified.

I hear knocking at the door. I say 'Tio devas esti Johano'. (Johano is expected.) Tio stands for the knocking at the door.

Then who did I see in the club last night? I say 'Tio certe estis Johano, mia frato'. The tio stands for the idea of someone seen (and mistaken for someone else). However, here, I could also reply 'tiu estis Johano, mia frato' and that couches it in terms of a particular individual.

Crude rule: 'tio estas mia patro' carries with it the idea of circumstances regarding the identification.

In other words when I say 'tio estas Johano/mia patro ktp', I am doing more than just pronomically referring to a particular individual.

I agree that this could be considered a subtle distinction. But it preserves the idea that tio essentially refers to some 'thing' rather than some person. The 'thing' is an idea or proposition linked to the person

sudanglo:

mi volas, ke li nun estu jam tute (mia filo), tio estas mia leĝa filo, kaj ke li estu nomata tiel, kiel mi, Dobĉinskij.

Surely the 'tio estas' in this quote is the equivalent of 'i.e.' (explanatory re-definition).

But let's consider some examples of where 'tio' is apparently used to refer to a person and ask what the tio represents.

I am called to the morgue to identify a body. I say to the police 'Tio estas mia patro.' Tio stands for the corpse (that thing), the body to be identified.

I hear knocking at the door. I say 'Tio devas esti Johano'. (Johano is expected.) Tio stands for the knocking at the door.

Then who did I see in the club last night? I say 'Tio certe estis Johano, mia frato'. The tio stands for the idea of someone seen (and mistaken for someone else). However, here, I could also reply 'tiu estis Johano, mia frato' and that couches it in terms of a particular individual.

... tio essentially refers to some 'thing' rather than some person. The 'thing' is an idea or proposition linked to the person

Thanks, sudanglo! That is very enlightening!

'tio estas' in the quote does seem to be equivalent to i.e. (id est, it is) meaning essentially '[to say] that is to say,...'

You make a compelling case that tio can appear to refer to a person, but really refer to something else such as a corpse (not quite a person anymore), knocking at the door, or an un/misidentified person (tio used to add emphasis to the un/misidentified aspect).

It seems to me that as a pronoun, tiu can inherently refer to a person (that one; where one means person). But tio refers to a thing. (The English language seems fundamentally opposed to admitting that people are things.)

In English, we have a similar situation with the pronouns he/she and it. The general rule is that you used he/she to refer to a person, but it to refer to a thing. You can break this rule by using it to refer to a person, but it has a dehumanizing effect which can be funny (It lives! after your 15-year-old son finally wakes up at 12:30 PM), or offensive (It puts the lotion on its skin or it gets the hose!).

While English doesn't have separate 'that' pronouns for things and people, we can approximate them with the phrases 'that thing' and 'that one/person'. And I think the effects of breaking the rule would be the same. 'That thing is my father' would imply that my father as a beast or a robot or something.

We can also break the rule in the opposite way. "She's a grand old ship" personifies a non-human thing to expresses affection for it.

Would you say that the usage of tiu and tio in Esperanto is similar to the usage of pronouns I've described in English?

This may not be completely related to what you're asking, but note that tio is also used as a dummy word to connect clauses:

-->Li plendis pri tio, ke lia hundo malsanas. - He complained that his dog was sick.
-->La kato dormas pro tio, ke ĝi lacas. ( = ĉar ĝi lacas) - The cat is sleeping because it's tired.

It is technically not wrong to omit tio from these kinds of sentences, but for some reason most Esperantists don't like to see forms such as pri ke or pro ke and prefer inserting the dummy tio. [1]

This is second nature for Slavic and German speakers, but looks funny in English. You don't say "He complained about that, that..."

I think it does, however, demonstrate that tio's functional role can vary from the English "that" or "that thing", even if there is much overlap.

If it helps any, you might think of tio as the most abstract demonstrative, and tiu as picking out a specific specimen or individual out of a set that is established in context. I like Sudanglo's "crude rule".

So Tiu estas mia patro means "That person, out of a range of people, is my father", whereas Tio estas mia patro just means "That (no particular context implied) is my father."

----
[1] A prominent exception is por ke, which seems to be more often than por tio, ke.

Tempodivalse:This may not be completely related to what you're asking, but note that tio is also used as a dummy word to connect clauses:

-->Li plendis pri tio, ke lia hundo malsanas. - He complained that his dog was sick.
-->La kato dormas pro tio, ke ĝi lacas. ( = ĉar ĝi lacas) - The cat is sleeping because it's tired.

It is technically not wrong to omit tio from these kinds of sentences, but for some reason most Esperantists don't like to see forms such as pri ke or pro ke and prefer inserting the dummy tio. [1]

This is second nature for Slavic and German speakers, but looks funny in English. You don't say "He complained about that, that..."

I think it does, however, demonstrate that tio's functional role can vary from the English "that" or "that thing", even if there is much overlap.

If it helps any, you might think of tio as the most abstract demonstrative, and tiu as picking out a specific specimen or individual out of a set that is established in context. I like Sudanglo's "crude rule".

So Tiu estas mia patro means "That person, out of a range of people, is my father", whereas Tio estas mia patro just means "That (no particular context implied) is my father."

----
[1] A prominent exception is por ke, which seems to be more often than por tio, ke.

I haven't come across that use yet. Thanks!

Pro tio, ke and pri tio, ke do sound a little odd to me, but not difficult; as in natural languages, you just accept it and go along with life.

Although, if we translate tio as 'that thing', maybe it sounds a little better.

-->Li plendis pri tio, ke lia hundo malsanas. - He complained about that thing, that his dog was sick.

You sort of need some 'thing' to complain about. Perhaps many Esperanto speakers aren't comfortable with 'that his dog was sick' being treated as a thing. I would normally say 'his dog being sick' in English.

Por ke is very familiar. The Spanish version is por que (edit: Oops, that should be para que)*; which sounds exactly the same. Perhaps por (for) is different from pro (on account of) for many speakers in a way that makes por ke more acceptable. I haven't really looked at prepositions yet.

*See http://www.spanishdict.com/translate/porque for porque used to mean 'so that'

Re: "dummy tio", it occured to me that this is often conveyed in English via "the fact that".

-->The fact that they're boycotting his products doesn't seem to worry him.
-->Tio, ke ili bojkotas liajn produktaĵojn, ŝajne ne zorgigas lin.

Note that even here you could omit tio and just as well start the sentence with Ke ili bojkotas...

Por ke + -u verb has been used since the earliest days of Esperanto; even in Zamenhof's era it did not have to include tio. You might argue that por ke means something beyond the mere sum of its parts - like the English "in order that", "so that".

But, prepositions are probably a good subject for a new thread. I don't want to digress from the initial topic.

Tempodivalse:Re: "dummy tio", it occured to me that this is often conveyed in English via "the fact that".

-->The fact that they're boycotting his products doesn't seem to worry him.
-->Tio, ke ili bojkotas liajn produktaĵojn, ŝajne ne zorgigas lin.

Would this change the agreement with a predicate noun?

Ke ili bojkotas liajn produktaĵojn, estas malbone.
Tio, ke ili bojkotas liajn produktaĵojn, estas malbona.

Btw, I am really glad that Esperanto's grammar can be concisely and precisely expressed in 16 simple rules.

eshapard:Por ke is very familiar. The Spanish version is por que; which sounds exactly the same. Perhaps por (for) is different from pro (on account of) for many speakers in a way that makes por ke more acceptable. I haven't really looked at prepositions yet.

Doesn't "por qué" mean "why"? "Por ke" means "so that", and I always reminded myself that it's not the same as "por qué" or "porque" (because).

Christa627:

eshapard:Por ke is very familiar. The Spanish version is por que; which sounds exactly the same. Perhaps por (for) is different from pro (on account of) for many speakers in a way that makes por ke more acceptable. I haven't really looked at prepositions yet.

Doesn't "por qué" mean "why"? "Por ke" means "so that", and I always reminded myself that it's not the same as "por qué" or "porque" (because).

Sorry for the confusion. What you said is correct. But I'm pretty sure that porque can mean either 'because' or 'so that' depending on the context (been a while since I studied it). Para que is probably the more correct way to say 'so that'.

Edit: Here's an example of porque used as 'so that'; not the most common use of porque and maybe not considered correct in all countries, but it's something I learned: http://www.spanishdict.com/translate/porque

nornen:
Would this change the agreement with a predicate noun?

Ke ili bojkotas liajn produktaĵojn, estas malbone.
Tio, ke ili bojkotas liajn produktaĵojn, estas malbona.

I checked the Tekstaro, but it has surprisingly few instances of Tio, ke in that context. I suspect most people prefer the straight-up Ke...

My immediate reaction is to say yes: tio demands -a. After all, we do this with other -o correlatives: Nenio estas nova sub la suno; io terura posedis lin. Why would tio be different? The modifier is modifying tio, and not the clause introduced by ke.

Consider also some Tekstaro cases:

"Nu, tio estas bona," opiniis Duponto.
Ajna diris, ke tio estas bona, sed...

Btw, I am really glad that Esperanto's grammar can be concisely and precisely expressed in 16 simple rules.

I do think I detect sarcasm.

I think it's obvious that 16 rules aren't going to explain every single facet of the language - this is why PAG and PMEG get to be over 500 pages.

Esperanto is quite complicated, if you get down to the details. The main difference is that there isn't any unproductive ballast like gender, declensions, etc. to distract the learner along the way, and oftentimes you can just think your way out of a problem if you have a hesitation - like I did above.

Just to get the Spanish straight:

para = por

para que = por ke
Trabaja para que coman. = Li laboras por ke ili manĝu.

por = pro / pri

por qué = kial
¿Por qué saliste? = Kial vi eliris?

porqué = kialo
Desconozco el porqué de sus acciones. = Mi ne scias la kialon de liaj faraĵoj.

por que = pro tio, ke / pri tio, ke
El estado vele por que los ciudadanos vivan en paz. = La ŝtato zorgu pri tio, ke la ŝtatanoj vivu pace.

porque = ĉar
Lo hice porque pude. = Mi faris ĝin, ĉar mi povis (ĝin fari).

Spanish is a bit more regular in this aspect. Instead of ĉar you simply say pro ke (in analogy to por ke), instead of kial you simply say pro kio (in analogy to por kio), and instead of kialo you say proo.