Citing the work that supports your research is both an ethical issue and a legal issue.
The ethics of citing is based on the scholarly tradition of giving credit for information and ideas that are not one's own. Science has a long tradition of acknowledging priority through citations. You have the ethical responsibility to cite all works that were used to support your research, to give credit to the earlier work and to provide a clear path for those who follow in your footsteps.
As so eloquently expressed by Isaac Newton in a letter to Robert Hooke in 1675 (Merton 1965):
If I have seen further it is by standing on ye sholders of Giants.
The legal issues of citing one's sources are codified in Copyrights, Title 17 of the U.S. Code, which describes the legal protections of authors' and creators' intellectual property rights.
[Merton, RK. 2006. On the shoulders of giants: a Shandean postscript. New York: The Free Press. p. 31]
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