The logic is that when you use "creer" in Spanish, you're not expressing doubt.

This is hard for us, because in English we have two kinds of "think," one unstressed, and one stressed.

Unstressed:  I think she's pretty. (no doubt is expressed; speaker is really expressing certainty).

Stressed:  I think she's pretty.  (doubt is expressed, speaker is not sure if she's pretty)

Spanish speakers only use "creer" to express certainty; i.e., only to correspond with the unstressed version above.  If the want to express non-certainty, they will not use "creer." 

An interesting side note:  you can use the verb "creerse" to mean "to mistakenly believe."  Because this expresses the speaker's opinion as well as the opinion of the subject, it's going to trigger the subjunctive: 

Este se cree que ella sea bonita.  This dude mistakenly believes that she is pretty. 

Thank very much for you answer.  I am still a little  confused because I think that the sentence “Creo que Jon está en la sala pero no estoy seguro” is possible in Spanish.

Also thank you for the point about creerse.  I think that you mentioned it on one of the lessons but I had forgotten it. 

Steve

ha ha, yes, that sentence is possible.  In the "creo que" clause, the speaker is using "creer" and therefore expressing certainty, and then when she says "no estoy seguro" he is expressing uncertainty. 

Remember, grammatical logic is different than formal theoretical logic.  Grammatial systems only have to be "logical" to the extent that meaning gets conveyed.  If the end result is a theoretical contradiction, that's irrelevant to the grammar! 

If our commucation were limited to only things that were theoretically logical in the formal sense, then I know some folks who would never speak....

Thank you again.  This is pretty heavy, grammatical logic verse theoretical logic. 

Buena leccion!  Gracias!

And is that belanova playing in the girls bedroom...

nice!

Cualquier manera, solo quiero decir que yo tambien me gusta belanova- pero creo que ese cancion esta llamada Rosa Pastel, no? Ademas, si ud quiere Belanova, check out Julieta Venega. Ambos son similar y bueno. Nos vemos!

MUCHAS MUCHAS MUCHAS Felicidades! Me siento muy orgullosa de ti! Sigue así y pronto llegarás a las lecciones avanzadas!

Julieta Venegas es mi cantante favorita. Escucha su album de Limón y Sal.

Dos correcciones.

MESES

AGRADEZCO