What allows to make a choice

For example, the body has the choice to walk on path A or B. Before the body starts walking, there must be thoughts of path A and B. There must also be a (mental?) decision/choice made to walk on one path or the other. What is it that allows to make choices? I have always been at loss to understand what that untouchable phenomenon is. What is it that let's me choose at all? I have trouble phrasing the question. So I will give another example. When someone offers me a marijuana joint, there are the options to accept or decline. Both feel just as far, just as easy to make. Sometimes I accept, sometimes reject. What phenomenon chooses? I've never been able to 'touch' that phenomenon directly. This question about changing habits is the same question as this one. EDIT: In another attempt to convey what is being asked (my apologies), there is now another question (this one with a bounty) https://buddhism.stackexchange.com/questions/18738/what-is-volition