The 1960s and 1970s were the era of one writer. Lem set the boundaries of the genre; Lem defined the genre; all young writers reflected Lem and competed with Lem. How could one author so completely dominate an entire literary category? It’s simple: he was quite simply a genius, with a mind that could fully display its powers precisely within the domain of science fiction.
First of all, he was very accomplished in the literary sense: he was able to tell stories, to build tension, to fascinate readers through narrative, suspense, and surprising denouements. Examples of this classical form are the Tales of Pirx the Pilot, or the novels Eden and The Invincible. These are good, hard science fiction, including space travel, contact with alien beings, and dangers brought by future technologies. Lem was equally capable of building other moods and atmospheres, for instance, terror (The Investigation) and romance (Return from the Stars).
Lem wore many other hats in his long career: mechanic, welder, WW 2 Resistance fighter, research assistant and so on. He was a polymath, and the same can be said for the astonishingly broad range of his fiction, essays and poems. Simply listing the sub-genres explored in his Science Fiction alone reads like an A-Z of library categories. Lem was a writers' writer who could turn his hand from reviewing imaginary novels to crafting whimsical but bitingly ironic cybernetic fairy tales and speculating about the difficulties of communicating with sentient oceans.