I want to have a society that is very isolated (physically) and has been for long enough that it doesn't really believe that there's anyone else out there. I also don't want the society to be in any way motivated to explore outside of its area. General background info about society (still up for change): The land where the people live is warm and fertile. They live in a river valley surrounded by mountains except at one end, where the river flows out. The society itself is matriarchal and has a polytheistic religion. There is one government, a monarchy lead by a queen but that also has a small aristocratic class. Most of the people there are farmers, fishing people, or artisans. There are other people who live not too far away on the other side of the mountains, but the occupants of the river valley are unaware of their presence.
Internal struggles would probably be the best reason for why the society would be unmotivated to explore.Exploration costs money and other government resources, which is why explorers like Christopher Columbus had trouble raising money for their proposed expeditions. If the government was facing instability, it would likely be reluctant to give any resources to an expedition, because there is no guarantee the expedition would prove fruitful. As AlexP pointed out in his comment though, it is incredibly unrealistic that a society would be unaware of another civilization living just beyond the mountains, even if they didn't explore. The only reason I could think of is that the surrounding area must be so dangerous, that it makes any communication difficult.