Did Einstein use his own reasoning?

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Did Einstein get his famous relativity theory from his predecessors (like Galileo, Newton, etc.) or from his own reasoning? ...

The Irish physicist George FitzGerald and the Dutch physicist Hendrik Lorentz were the first to suggest that bodies moving through the ether would contract and that clocks would slow. This shrinking and slowing would be such that everyone would measure the same speed for light no matter how they were moving with respect to the ether, which FitzGerald and Lorentz regarded as a real substance.

But it was a young clerk named Albert Einstein, working in the Swiss Patent Office in Bern, who cut through the ether and solved the speed-of-light problem once and for all. In June 1905 he wrote one of three papers that would establish him as one of the world's leading scientists--and in the process start two conceptual revolutions that changed our understanding of time, space and reality.

In that 1905 paper, Einstein pointed out that because you could not detect whether or not you were moving through the ether, the whole notion of an ether was redundant.

No, Einstein's comments about the aether were essentially the same as what Lorentz published in 1895. Whether the aether is a "real substance" is a philosophical question, and you get different answers even today. Einstein later said that he believed in the aether, but not aether motion.

As a historical matter, Einstein's 1905 paper did not change our understanding of time, space and reality.

If you wanted to live longer, you could keep flying to the east so the speed of the plane added to the earth's rotation.

Einstein had a similar comment in his 1905 paper, but it was wrong because it fails to take gravity into account.

This unease continued through the 1920s and '30s. When Einstein was awarded the Nobel Prize in 1921, the citation was for important -- but by Einstein's standards comparatively minor -- work also carried out in 1905. There was no mention of relativity, which was considered too controversial.

No, there was no controversy about the 1905 special relativity. Special relativity became widely accepted in about 1908 because of theoretical work by Lorentz, Poincare, and Minkowski, and because of experimental work that distinguished it from competing theories.

No one wanted to give Einstein the Nobel prize for special relativity because no one thought that he created the theory or the experimental work.

Some of the other answers mention Lorentz and Poincare as having discovered special relativity.