How do we know that we have free will?

Question

Where in the torah does it state that humans have, and are entitled to in perpetuity, free will?

Answer 1

The verse probably most commonly cited is Devarim 30:19 (translation from Mechon-Mamre):

הַעִדֹתִי בָכֶם הַיּוֹם, אֶת-הַשָּׁמַיִם וְאֶת-הָאָרֶץ--הַחַיִּים וְהַמָּוֶת נָתַתִּי לְפָנֶיךָ, הַבְּרָכָה וְהַקְּלָלָה; וּבָחַרְתָּ, בַּחַיִּים--לְמַעַן תִּחְיֶה, אַתָּה וְזַרְעֶךָ

I call heaven and earth to witness against you this day, that I have set before thee life and death, the blessing and the curse; therefore choose life, that thou mayest live, thou and thy seed.

The Torah here implies that a Jewish person has a choice whether to follow God's laws and "live" and "be blessed", or on the other hand rebel thus choosing "death" and "curse". The last phrase "you and thy seed" implies, as you say, our entitlement "in perpetuity", to this free will.

Rambam (Mishna Torah Hil. T'shuva 5:1) uses a different source (my own translation):

רשות לכל אדם נתונה אם רצה להטות עצמו לדרך טובה ולהיות צדיק הרשות בידו. ואם רצה להטות עצמו לדרך רעה ולהיות רשע הרשות בידו. הוא שכתוב בתורה הן האדם היה כאחד ממנו לדעת טוב ורע. כלומר הן מין זה של אדם היה יחיד בעולם ואין מין שני דומה לו בזה הענין שיהא הוא מעצמו בדעתו ובמחשבתו יודע הטוב והרע ועושה כל מה שהוא חפץ ואין מי שיעכב בידו מלעשות הטוב או הרע וכיון שכן הוא פן ישלח ידו

Choice is given to every person. If he wishes to align himself with the path of good and be righteous, the choice is his. And if he wishes to align himself with the path of wickedness and to be evil, the choice is his. This is what is written in the Torah (Bereshis 3:22), "Behold, the man is become as one of us, to know good and evil." Meaning, this [human] species is unique in the world and there is no other species like it in this respect that he with his own intellect and reasoning knows the good and the bad, and then does whatever he desires without anyone forcing him to the good or the bad. Therefore, "lest he put forth his hand..."

Answer 2

Devarim 10:12 says:

1. And now, O Israel, what does the Lord, your God, demand of you? Only to fear the Lord, your God, to walk in all His ways and to love Him, and to worship the Lord, your God, with all your heart and with all your soul,

Rashi says:

Only to fear [the Lord your God,…]: Our Rabbis derived from this verse [“And now… what does… God demand of you”] that everything is in the hands of Heaven except the fear of Heaven (Ber. 33b).

Rashi on the quoted Gemara (Berachot 33B) says (my translation):

הכל בידי שמים. כל הבא על האדם ביד הקב"ה הוא כגון ארוך קצר עני עשיר חכם שוטה לבן שחור הכל בידי שמים הוא אבל צדיק ורשע אינו בא על ידי שמים את זו מסר בידו של אדם ונתן לפניו שני דרכים והוא יבחר לו יראת שמים

Everything is in the hands of Heaven: Everything that happens to a person is in the hands of G-d, for example if he'll be tall or short, poor or rich, wise or foolish, white or black. All the aforementioned is in the hands of Heaven, but whether one is righteous or wicked is not in the hands of Heaven. This G-d gave over to man and placed before him two paths. He must chose fear of Heaven.