Mary gives out two poisoned letters.
One to Sherlock, with chloroform, to try incapacitate his mind and body. It works for a bit,
One to John, with saccharin, to try to incapacitate his heart, his emotions. It doesn’t seem to work, at all. He doesn’t do what she asks and in Morocco admits that he was the one tracking her and that he no longer even likes her,
The first one is from her heart and the second one is from her brain. Circles are symbolic of feelings and rectangles are symbolic of thoughts/reason.
We see a circular effect around Sherlock to show that the drug is taking effect but this also shows that this is a message from Mary’s true feelings: she wants him out of the picture, immobilised, impaired.
We see two staggered rectangular windows throwing their shape up against the wall behind John as he reads Mary’s letter. These are like the rectangles in Mycroft’s office. Mycroft is often associated with these rectangles, he’s the one who truly believes (apparently) that reason is to be exalted above everything else.
The circular motif associated with Mary’s letter to Sherlock can also denote that this is an emotional journey for Sherlock. Discovering who Mary is, trying to protect her and John; this, like Mycroft says, is about sentiment.
For John, reading Mary’s letter may be an intellectual exercise. He may not have any trust left for her. He says he, ‘used to’, like her, so he’s saying he doesn’t even like her, any more. This letter may not be affecting him on an emotional level other than to consider what type of negative consequences it might mean for himself and his baby. These rectangles could be about John trying to figure out how Mary thinks, what her next move may be. What his next move ought to be. Maybe the rectangles are like a chess board on which he contemplates his strategy.
The rectangles associated to Mary’s letter to John may be a sign that she and Mycroft continue to work together despite the latter’s denials. She may have been hired by him to, ‘keep a weather eye’, on him between TRF and TEH and then like Mary says, ‘something went wrong’. And just like in Georgia, that something appears to have been Mary.
