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A definition, offered by Rod Serling, is that "science fiction makes the implausible possible, while science fantasy makes the impossible plausible." The meaning is that science fiction describes unlikely things that could possibly take place in the real world under certain conditions, while science fantasy gives a veneer of realism to things that simply could not happen in the real world under any circumstances. Another interpretation is that science fiction does not permit the existence of supernatural elements; science fantasy does. Even the usage of this definition is difficult, however, as some science fiction makes use of apparently supernatural elements such as telepathy.
For many users of the term, however, "science fantasy" is either a science fiction story that has drifted far enough from reality to "feel" like a fantasy, or a fantasy story that is attempting to be science fiction. While these are in theory classifiable as different approaches, and thus different genres (fantastic science fiction vs. scientific fantasy), the end products are sometimes indistinguishable.