In the political realm, "possible" often refers to what is deemed able to be done, given current conditions. Thinking outside of the possible brands one an idealist. That raises a slew of questions:
Who gets to decide what politics are possible?
How is a horizon of the politically possible established, stabilized, and reproduced?
More hopefully, what prompts political horizons to expand, to be transformed?
How does the "impossible" become possible?
See also PRAGMATISM, tyranny of.