Using stream-of-consciousness narration, Woolf is able to depict the thoughts of individual characters. This allows each character to think differently.
One of the themes of the text is that people, while physically sharing space, can be experiencing quite distinctly different internal realities. This fact is treated with profound importance in this novel and others by Woolf and Faulkner and is, by extension, one of the more important themes of modernism in literature.
"Subjective realities" are fully on display in Mrs. Dalloway, with each character experiencing a flow of thought that creates a unique sense of 1) time, 2) history and 3) psychological experience. These ideas are present in many - if not all - of the characters in the novel. The extremes of subjective reality and psychological isolation can be seen in Mrs. Dalloway and Septimus.
Clarissa moves away from isolation toward an acceptance of life in all its puzzling complexity; Septimus moves ever deeper into isolation and finally suicide.
The implications of subjective and personal realities are explored through these characters in the novel. While there is clearly a dark side to Woolf's vision of psychological isolation, there is also a suggestion that through generosity and positive feelings (love), a person can find the strength to overcome the isolating effect of the realization that one's reality is one's own.
The novel seems to ask if people can truly communicate and connect if each is enclosed within his or her own consciousness. Whether the novel resolves this issue, or merely explores it, is for each reader to decide.