It was taught. "Your hand," written with the (superfluous letter) "hay," (at the end of the word in Exodus 13:16) indicates the left hand.

Others say, "Your hand," (that superfluous "hay") includes a man that has but the stump of the arm.

Another [Baraita] taught: One that has no [left] arm is exempt from tefillin. Others say, ‘Your hand', includes a man that has but the stump of the arm.

Our Rabbis taught: A left-handed man puts his tefillin on his right hand for that is his left.

But it has also been taught that he must put it on his left hand which is also the left for all people!

The latter was taught of a person who is ambidextrous.

A Tanna in the school of Manasseh taught: Upon your hand, that is, on the biceps muscle; between your eyes, that is, on the skull.

On what part?

It was said in the school of R. Yannai, Where the skull of a baby is still tender.

Pelemo enquired of Rabbi, If a man has two heads on which one must he put the tefillin?'

‘You must either leave', he replied, ‘or regard yourself under the ban (a form of excommunication placed on pain-in-the-neck, impudent disciples).'

In the meantime there came a man [to the school] saying, ‘I have begotten a first-born child with two heads; how much must I pay the priest (for redemption of the first born)?'

The Master said, ‘Upon your hand, that is, on the biceps muscle.'

From where is this derived?

Our Rabbis taught: Upon your hand, that is, the upper part of the hand.

You say it is the upper part of the hand, but perhaps it means actually upon the hand?

Since the Torah ordains that one must put tefillin upon the hand and also upon the head, as in the latter case it is to be upon the upper part of the head so in the former it is to be upon the upper part of the hand.

R. Eliezer says, This is unnecessary; for the verse says, ‘And it shall be for a sign for you upon your hand,' implying that the sign shall be for you but not for others.

R. Isaac says, This too is unnecessary; for it is written, And you shall lay up these My words in your heart . . . and you shall bind them, implying that it must be placed over against the heart.

R. Hiyya and R. Aha the son of R. Ivia used to place it exactly over against the heart.

R. Ashi was once sitting before Amemar. The latter had an injury on his arm and his tefillin were exposed; whereupon R. Ashi said to him, Does not the Master hold ‘it shall be for a sign for you but not for others?'

That, he replied, was stated only to indicate the place, namely, where it is a sign for you only.

From where is it derived that it must be upon the upper part of the head?

Our Rabbis taught: ‘Between your eyes,' that is, the upper part of the head.

You say it is the upper part of the head, but perhaps it means actually between the eyes?

It is written here, ‘Between your eyes,' and it is written there, "Nor make any baldness between your eyes for the dead;" as in the latter case it means the upper part of the head where baldness can be made, so in the former case too it means the upper part of the head where baldness can be made.