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"Letras" is a form of "letras", a plural noun which is often translated as "arts". "Filosofía" is a noun which is often translated as "philosophy". Learn more about the difference between "letras" and "filosofía" below.
las letras(
leh
-
trahs
)A plural noun indicates that there is more than one person, place, thing, or idea.
1. (education)
2. (short text)
a. line 
Les pondré unas letras agradeciéndoles la invitación.I'll drop them a line thanking them for the invitation.
la letra
A feminine noun is almost always used with feminine articles and adjectives (e.g., la mujer bonita, la luna llena).
a. handwriting 
Siempre se ha dicho que los médicos tienen muy mala letra.It's always been said that doctors have really sloppy handwriting.
b. writing 
¿Qué dice aquí? No te entiendo la letra.What does it say here? I can't read your writing.
5. (music)
6. (finance)
a. installment  (payment) (United States)
Regionalism used in the United States
Hoy pagué la última letra del televisor.I paid the last installment for the television today.
b. instalment  (payment) (United Kingdom)
Regionalism used in the United Kingdom
Todavía nos quedan cuatro letras para acabar de pagarlo.We still have four instalments left to finish paying for it.
c. bill  (document)
Fueron tiempos difíciles. Continuamente nos llegaban letras que no podíamos pagar.They were difficult times. We continuously received bills we couldn't pay.
a. literal meaning 
Ateniéndose a la letra de la ley, el consumo de esta droga es ilegal.Sticking to the literal meaning of the law, the use of this drug is illegal.
la filosofía(
fee
-
loh
-
soh
-
fee
-
ah
)A feminine noun is almost always used with feminine articles and adjectives (e.g., la mujer bonita, la luna llena).
a. philosophy 
Lourdes es apasionada de la filosofía y la religión del siglo XVII.Lourdes is passionate about 17th-century philosophy and religion.
a. philosophy 
Mi mamá tiene una filosofía de vida muy positiva.My mom has a very positive philosophy of life.