Lyle Denniston comments for Constitution Daily:
It probably was an act of legal genius that the Founders who wrote the Fourth Amendment made its protection of personal privacy apply only to “unreasonable searches and seizures.” The word “unreasonable” is obviously very open ended: what is reasonable to one viewer may be quite unreasonable to another. But, while that word can make interpretation difficult for judges, lawyers and academics in specific situations, it still has a common sense ring to the ears of ordinary people.
Essentially, what the Fourth Amendment always protects is what might be called those aspects of an individual’s private life that he “intends to keep to himself,” as one Supreme Court Justice once put it. But when the late Justice John Marshall Harlan wrote that in 1967 (the first time any Supreme Court opinion spoke of a “reasonable expectation of privacy”) he added to it a second part: in order to get the Fourth Amendment’s protection, that notion of privacy must be what the rest of society would accept as “reasonable.”
This gives the Fourth Amendment a personal cast. But it has to exist within a wider social context. As either changes, over time, the Fourth Amendment might be understood to mean something different.
Of course, there are judges and academics who insist that the Fourth Amendment means only what it meant in principle when the Bill of Rights was added to the Constitution in 1791, and that it does not adapt to the times. . . .