There are 3 major reasons why sci-fi is the ideal medium to discuss philosophical ideas: sci-fi engages the reader’s intellect, is about big ideas, and is the literature of change.
Sci-fi engages the reader’s intellect
Sci-fi is not a relaxing read. It can be quite mentally taxing because the writer has often created an elaborate, advanced or alien world based on science. Magical realms can be accepted without question, but scientific worlds must make some sort of sense. The reader’s brain must act like a computer, taking the writer’s code and reconstructing the world in realtime as he reads. The reader must often fill in technological or historical gaps himself because had every detail been written, the sci-fi work would easily be twice as long.
Sci-fi is about big ideas
“The future. Space travel, or cosmology. Alternate universes. Time travel. Robots. Marvelous inventions. Immortality. Catastrophes. Aliens. Superman. Other dimensions. Inner space, or the psyche. These are the ideas that are essential to science fiction. The phenomena change, the basic ideas do not. These ideas are the same philosophical concepts that have intrigued mankind throughout history.” – Kate Wilhelm
As I have written elsewhere, science is the direct descendent of philosophy. Ideas that were conjectures and mere possibilities in philosophy are rapidly becoming realities with science. Philosophy has not become irrelevant; rather, it has been strengthened and given form. In its endless quest for the ultimate nature of reality, science inevitably raises philosophical questions that have endured since man became self aware.
Sci-fi is the literature of change
“That’s really what SF is all about, you know: the big reality that pervades the real world we live in: the reality of change. Science fiction is the very literature of change. In fact, it is the only such literature we have.” – Frederik Pohl
Science constantly pushes the boundaries of our knowledge, and sci-fi is the only medium that consistently strives to stay one step ahead. Before the days of organized science, philosophy was man’s way of exploring cutting edge topics. In our scientific world, sci-fi is the new philosophy.
Philoscifi 
See this Wired article. I wouldn’t say that sci-fi is the last bastion of philosophy, but it’s an interesting read.
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Here’s another good article I stumbled upon recently from io9 that argues that SF grew out of philosophy. Good read and discussion.
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I respectfully disagree sci fi is the only consistant local for philosophical ideas and mind opening literature look to Wolfe,”the Melville of our times” ,he asked us to remain smart , to look and feel for the deeper layers of the books he graces our times with..the questions of all the intended clues planted everywhere for the serious reader…the sci if readers..it takes more than a teaspoon of brains to read and understand this great work .Le
Guins genius brings us worlds of thoughts,emotion,wisdom, and depth. Revel in these writings,they are forbidden and ridiculed in police states for they fear sci -fi.”throw of the shackles of tyranny, read!”
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