This is a follow-up to your response regarding automatic light timers where you also addressed motion detector lights. We recently moved into a ground floor apartment with a small backyard that we have not been using on Shabbat because our non-Jewish neighbor with a similar yard has a motion detector light that turns on whenever we enter our yard. (We are in an urban area and the light is for security purposes.) Two questions: 1. Is there any way we can use the yard because even though the light has always been on and we suspect it will be this way next time we do not know this with 100% certainty as the switch is controlled by the neighbor. 2. Is there any difference for Yom Tov because we would like to build a Sukah in the yard.

According to some rishonim, and some present day authorities, a necessary result of our actions that is not desired (p'sik reisha d'lo neicha lei) is allowed. Some extend this even to results we don't care about. According to that opinion, since you don't care about the light, it would be permissable to walk where you might set it off. (This is the opinion of Rav Ovadiah Yosef, as well as may Ashkenazi poskim) Two further mitigating factors are that you do not know %100 that the light will go on, and that you want to use the yard for a sukkah. If the light is not incandescent, according to many poskim electricity itself is a rabbinic category, and the situation becomes further eased. I would have no difficulty allowing the use of the sukkah, but would try to get the neighbor to use a non incandescent bulb, and then it would be permissable to go into the yard all year long.