RATIONALISTIC REFERENCE FROM VALMIKI RAMAYAN;
AYUDYA KAND, CHAPTER 108,VERSE 14-15-16-17.
A Brahman named Jabali tries to persuade Rama to accept the kingdom by advocating the theory of nastikas [non believers]. 

aShTakaa pitR^i daivatyam ity ayam prasR^ito janaH |
annasya upadravam pashya mR^ito hi kim ashiShyati || 2-108-14
14. ayam= these; janaH= people; prasR^itaH= say; iti= that; aS

Takaa= the eight day; pitR^idaivatyam= should be given up to sacrifices for the spirits of our ancestors; pashya= see; upadravam= the waste annasya= of food; kim= what; mR^itaH= (will) a dead man; ashiSyati= eat?
"These people say, 'The eighth day should be given up to sacrifices for the spirits of our ancestors.' See the waste of food. What will a dead man eat?"

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yadi bhuktam iha anyena deham anyasya gagcCati |
dadyaat pravasataH shraaddham na tat pathy ashanam bhavet || 2-108-15
15. yadi bhuktam= If food eaten; anyena= by one; iha= here; gachchhati= reaches; anyasya= another's; deham= body; shraaddham= (then let) a sacrifice; dadyaat= be offered; pravasataH= for those who are setting out on a distant journey; tat na bhavet= will it not become; ashanam= a food; pathi= on their path?
"If food eaten by one here, reaches another's body, then let a sacrifice be offered for those who are setting out on a distant journey. Will it not become a food on their path?"*

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daana samvananaa hy ete granthaa medhaavibhiH kR^itaaH |
yajasva dehi diikShasva tapaH tapyasva samtyaja || 2-108-16
16. yajasva= perform sacrifices dehi= distribute gifts; diikSasva= consecrate yourselves; tapyasva= practise; tapaH= austerity; samtyaja= and renunciation; ete= there granthaaH= writings; kR^itaaH= are composed; medhaavibhiH= by learned men; daana samvananaaH= for the sake of inducing others to give.
"Perform sacrifices, distribute gifts, consecrate yourselves, practice austerity and renunciation' - These writings are composed by learned men for the sake of inducing others to give."

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sa na asti param ity eva kuru buddhim mahaa mate |
pratyakSham yat tad aatiShTha parokSham pR^iShThataH kuru || 2-108-17
17. mahaamate= O, the highly wise!; saH= you as such; kuru= arrive at; buddhim= a conclusion; naasti ityeva= that there is nothing; param= beyond this universe; aatiSTha= give precedence; tat= to that; yat= which; pratyakSam= meets the eye; pR^iSThataH kuru= and turn your back; parokSam= on what is beyond our knowledge.
"O, the highly wise! Arrive at a conclusion, therefore, that there is nothing beyond this Universe. Give precedence to that which meets the eye and turn your back on what is beyond our knowledge."
*The same exposition has been provided by Charvaka in Charvaka darshan.