How did Avraham discover that God exists? Was because of logic or tradition since Adam? If was by logic, why do we call him Ivri (from Ever, his grandfather). Please bring sources.
There is a midrash in Bereshit Rabbah 39 where Avraham deduces the existence of God who created the world, and only then did God call out to Avraham lech l'cha. A summary of the midrash from my notes after a class:
So according to this midrash, Avraham was inquisitive enough to deduce the existence of, and then seek out, the Master of the Universe.
Another midrash (B'reishit Rabbah 42) offers a few different explanations for the name Ivri (h/t Menachem). One of these, attributed to R. Nechemiah, is that it comes from Eber, from whom Avraham learned the monotheistic tradition (passed down from Shem). That interpretation contradicts the midrash above. The other two explanations there are not related to this. R. Yehudah says that it is because Avraham worshiped one God unlike those around him, and others understand it geographically, because Avraham "crossed over" and wasn't a native.
No doubt, tradition played an important part of Abram's first obedient response to God's command to get up and move from Haran, where he and his father's family had moved after leaving polytheistic Ur of the Chaldeans (Bereshit 11:31). Abram was not born into a "religious" vacuum, even though his father may not have been a believer in YHWH, the one true God.
Moreover, Abram was an intelligent man, capable of rational thought and logic. On more than one occasion he asked God intelligent and penetrating questions (see, e.g., Bereshit 15:2-3 and ff.). Then too he was a prosperous, blameless man (though not a sinless man) with a spotless reputation who lived in the midst of a culture of polytheism for much of his life. He had "smarts" enough to go against the four kings of Genesis 14 and defeat them, after which he refused to receive a gift from the king of Sodom, saying
God would not have appeared to Abram had he not been both a believer in God and a sensible believer at that! When YHWH made the promise to Abram to make his descendants as numerous as the stars of heaven, the Tanakh said, "And [Abram] believed in the LORD; and He counted it to him for righteousness." (Bereshit 15:6).
I'd like to think that Abram had heard, through the lineage of Shem (Bereshit 11:10 ff.), the story of the one true God who had by His word brought the created universe into existence ex nihilo, and that the story had been passed down from generation to generation, sometimes being met with faith and at other times not.
Even if Abram had not heard of the one true God, and there is no particular reason to believe he had not, there is such a thing as "the witness of God in nature," or "natural revelation," as some theologians call it.
In the Tanakh, the Psalmist David put it this way,
In other words, unbelieving men and women are without excuse. What is known of God is clearly evident to them; they choose, however, to suppress the truth and do what they consider natural by following their own lusts. The prophet Isaiah called such people "sheep," each of whom has "gone astray" and "turned to his own way" (Yeshayahu 53:6). If we all are honest with ourselves, we all (believers in the one true God and unbelievers alike) tend to stray from what we know to be right and choose to do what God commands us not to do. (See Tehillim 53:1-3.)
One of the "arguments" agnostics and atheists use as a smokescreen (from my perspective, anyway) is the old
In conclusion, then, Abraham, the great patriarch of faith, knew in his heart that there was one true God, the Creator and the Sustainer of all the earth. He knew, moreover, that God is a just God. That is why in Bereshit 18:25, he asked
All I can say to that is, Amen!