What is the grammatical explanation of “sí” in “entre sí”?

Question

This means "they" mix together (or each other):

mezclan entre sí

In my knowledge, sí is to acknowledge or affirm. Please explain the effect or function of sí in this case. Does it create "mutual" relations among the words or objects?

When I want to say "gravity pulls the planets among them", can I say something like 'las planetas se sacalan entre sí'?

Answer 1

"Sí" is also the reflexive form of the third person pronouns (él, ella, ellos, ellas), which must always be preceded by a preposition. This "sí" is completely unrelated (has different etymology) to the word "sí" meaning "yes".

Reflexive means that the subject applies an action to themselves. A possible translation in some cases would be "preposition+himself/herself/themselves". The word "mismo/misma" is often appended to emphasize the reflexiveness of the action, but it is not mandatory. For example:

• Dijo para sí (mismo)... / He said to himself...

• Lo alejó de sí (misma) / She pushed him away from her(self)

• El frío en sí no me molesta: me molesta no poder ir a la piscina (The cold weather itself does not bother me: what bothers me is not being able to go to the pool)

The construction "entre sí" means therefore "among themselves", which might more naturally be translated as "each other" or "together" in some cases:

• Los planetas se atraen entre sí (The planets attract each other)

Answer 2

This is si as the third person reflexive pronoun both singular and plural. It also work for Usted(es).

I would have said that a translation might be they (or you plural polite form) mix amongst themselves (or yourselves) but more context might help to clarify.