The Mishna on today’s daf teaches about situations where people can forgo the mitzva of sukka. According to the Mishna, sheluhei mitzva – people occupied with performance of a mitzva – are not obligated in the commandment of sukka. Also included on this list are people who are ill, together with their caretakers. Finally, the Mishna teaches that only a formal meal needs to be eaten in the sukka; a snack can be eaten outside of the sukka.
The idea that ha-osek be-mitzva patur min ha-mitzva – that someone engaged in a mitzva is free from his obligations in other commandments – is derived from a passage in keri’at shema. We read in shema that we are obligated to discuss the words of shema – be-shivtikha be-veitekha u-velekhtekha va-derekh – when you are sitting in your home and when you are walking on your way. The emphasis on “your home” and “your way” teaches that when you are occupied in matters that are not your own choice, that is to say, matters that you are obligated to be involved with – i.e. mitzvot – then you are not obligated in shema, nor, for that matter, are you obligated in other mitzvot.
One point that is not clear, and is the subject of debate among the poskim, is whether we apply the rule of ha-osek be-mitzva patur min ha-mitzva only in a case where performing the additional mitzva will adversely affect fulfillment of the first mitzva (for example, if spending time looking for a sukka will limit the amount of time the travelers will be able to devote to their travel, making fulfillment of the mitzva take a longer time). Others argue that involvement in a mitzva simply creates a situation whereby a new obligation cannot be imposed on the person who is already occupied with a mitzva. According to this view, even if the second mitzva can be done with no additional strain or effort, the person is still not obligated to do it.