"The Stranger: An Essay in Social Psychology" by Alfred Schutz is a notable sociolical essay that picks up on Georg Simmel's notion of the stranger. What is of interest to Schutz is the stranger's integration and assimilation into society. Schutz is referring to all types of such integration into a new group like moving to a new city or joining an exclusive group.
In order to understand Schutz's understanding of the stranger we must first describe how he considers regular members of groups. According to Schutz the member of a group is situated within it and therefore has a set of layers of information which are not completely coherent, are only partially clear and non consecutive. This is all from an objective point of view, but if we turn to the subjective view of the individual which belongs to a group all of these "problem" of knowledge are transparent since groups provide their members with frameworks and groups assumptions that "iron out" all these problems. This is what Schutz calls "thinking as usual", the self evident thought that things will continue the way they are, the inherited knowledge is still applicable, that events are manageable under current knowledge and the thought that others will interpret events in the same manner we do. According to Schutz, if one of these terms fails to be satisfied that particular group will have to undergo change.
The stranger, on the other hand, does not have to share the aforesaid base assumptions. Everything which is obvious to the group could be questioned by the stranger who lacks the shared ground assumptions. For Schutz this is important since it justifies outside inquiries into societies, enabling to realization of things they cannot see about themselves.
The stranger for Schutz is new to the group. He can share in its present and future but never in its past. His "thinking as usual" is different, and even if he acquires the group's mode of thought he will always have a different base to built it on.