Shabbat 2: Private and Public Domians, Two Equals Four

We begin Massechet Shabbat with a Mishna dedicated to the concept of yitziyot, carrying out from the public domain to the private domain, an activity forbidden by the laws of Shabbat . Thre are eight cases in question.  Four of these cases are two basic actions from the perspecitive of a person inside of a private domain and another two actions comprise four cases from the outside from the perspective of a person outside.

1. A poor person stands in the public domain and a homeowner stands within.  The poor person lifts and hands an item into the public domain, into the hands of the home owner.  Or the poor person reaches his hand into the private domain and takes an item from the homeowner, carrying it to the public domain.  In both cases, the poor person has transgressed and the homeowner is exempt.
2. The homeowner lifts an item in the private domain, reaches into the public domain and puts the object into the hand of the poor person.  Alternately, the homeowner reaches his hand into the public domain and puts it into a poor person's domain.  In both cases, the homeowner is liable and the poor person is exempt.
3. The poor person reaches into the private domain and the homeowner places an object in his or her hand.  The poor person carries it out into the public domain.  Each performed only half of the prohibited labour and so both are exempt.
4. If the homeonwer extends his hand into the public domain and the poor person takes an object into the public domain or the poor person places into the homeowner's hand and s/he carries it into the private domain.  Both have only involved half of the labour, and so they are both exempt from any punishment.

Our Gemara notes that we must bring a sin-offering when we are a situation where there are two that comprises four.  the first cases are prohibited because we are not to to perform a specific action in the future.  Two more cases are added: one who swore that the he performed a specific action in the past and one who said that he did not practice an an action any more than necessary.  

When it comes to ritual purity, we also come across the notion of two equals four.  They regard unwitting transgressions:

1. One who was aware and then forgot this he was ritually impure, then ate consecrated meat or entered the Temple.  
2. One who was unaware s/he was about to enter the Temple and then entered it.  

Symptoms of leprosy are also two that comprise four:  In Leviticus 13, we learn that baheret and se'et are two signs of leprosy.  However two additional secondary signs of affliction included marks that are not as white.  Instead they are understood to be derivatives of the signs of leprosy.  

Carrying out is a a primary prohibition of Shabbat.  Massechet Shevuot also mentions this rule of two equals four.  The rabbis discuss how our examples demonstrate immediate action, illumination the rabbis' understandings of the 'domains of Shabbat'.  Then, when considering what is inside and what is outside, the rabbis wonder if these might be eight or twelve or even sixteen cases due to the individual actions taken.