We come frequently across a phrase in later Avesta, namely ” átarsh púthrä ahúra-hä mazdáw.” This phrase has been commonly translated as ” fire the son of Ahúra Mazdá.” The word translated as “son” is “púthr/púthrä.” Modern farsi words such as “púr/son” and “partoo/reflection/rays of light,” are derived from the same Avestan root.  “Púthr” could be compared with latin “filius,” words such as “fruit” and “felix” are all derived from the same ancient root namely: “to yield, produce, bear fruit, grow, be auspicious, happy and fruitful.”
Translation of “púthr” as son, seem not to do justice to the original Avestan. In my opinion, translating “púthr” as PRODIGY is a much more accurate and closer choice. In the verses of the prophet, we encounter the phrase “töí áthrém,” or “Your Pure Blazing Fire/Flame,” instead of the aforementioned formula. Compare Avestan “töí” with Old.English. þu, Ger. du, Old.Norse. thu, Goth. þu, Latin. tu, Irish. tu, Welsh ti, Greek. su, Lithuanian. tu, Old.Church.Slavonic. ty, Sanskrit. twa.    
  Fire appears as gathic “áthra/áthrö,” later Avestan “átarsh” and modern farsi  “átash/ázar.” They are the same as Old.French. ether, Latin. aether or “a purer form of fire or air,” Greek “aither” from “aithein;” “to burn, shine,” Sanskrit. inddhe “burst into flames,” Old.Irish. aed “fire,” Welsh aidd “heat, zeal,” Old.High.German. eit “pyre.” In addition, “edifice” or originally “a place with a hearth” is from the same base.
áthra/áthrö refers to a pure form of energy or fire that constitutes the substance of the universe and the stars.  It is called the púthr of Ahúra Mazdá, because creative energy or light is the closest reflection of GD. Notice the beautiful connection between the concept of growth, rays of light, delight and pure, transformative, blazing energy.
To best understand the idea of áthra/áthrö as the purest refelction of Gd or the prodigy of Ahúra Mazdá; a look at the idea of fire in  Heraclitus Philosophy is most helpful.   Heraclitus was born to an aristocratic family in Ephesus, present-day Efes, Turkey. Ephesus had been part of the Ancient Zoroastrian Persian Empire since 547 and was ruled by a satrap or Persian Vice-Roy.
Contrary to the static worldview of the Greeks at the time; Heraclitus looked at everything being in the state of permanent flux, hence, reality being merely a succession of transitory states. He told people that nothing is the same now as it was before, and thus nothing what is now will be the same tomorrow.  According to Heraclitus all things come to be in accordance with “LOGOS” (literally, “word,” “account,” “measure,” “formula” or “reason.”) Heraclitus used “fire” in lieu of LOGOS, and declared it as the origin of all things. This connection bewteen logos and fire; shows uncanny resemblance to the connection between “the auspicious spirit of wisdom” (speñtá mainyü ) and fire in the poetic gathas of the prophet, see Yasna 36.2. 
All things are an interchange for fire, and fire for all things. Heraclitus held that fire in lieu of LOGOS is the primordial energy out of which everything else arises. Fire is the origin of all matter; through it things come into being and pass away.
When Heraclitus speaks of God, he doesn’t mean the Greek gods. Instead he thinks that God is living, fiery element  in every soul and even in every material thing on earth, the common-sense notion of a ‘thing which gives rise to the other elements and thus to all things.
The notion for such creative flame is also present in the Moslem Koran, where GD sates that he has made the angels out of a fire without smoke.
 
ardeshir