Writers put words together in unique ways that express ideas. Is plagiarism — taking those ideas or words without attribution — inherently wrong? In a recent column, Stanley Fish argues that it is not (link). We are all influenced by the words and ideas of others. R.R. Reno argues, in response to Fish, that the issue is less one of theft than of creating a false impression of ability (I, II). In other words, is plagiarism an issue of theft of intangible property or lying? What does Jewish law have to say about it?
Plagiarism is forbidden according to Jewish law. The Magen Avraham (156:2) quotes a midrash (somewhat ironically sourced incorrectly as a Gemara) that someone who quotes a saying without noting its source violates a prohibition. R. Shlomo Kluger (Chokhmas Shlomo glosses, ad loc.) writes in agreement, as does the Noda Bi-Yehudah (Responsa, Orach Chaim vol. 2 no. 20). What, specifically, is the prohibition?
R. Moshe Feinstein (Iggeros Moshe, Choshen Mishpat vol. 2 no. 30) writes that students in school are forbidden to copy off each other because it is geneivas da’as, a misrepresentation. It gives the incorrect impression that the student acquired knowledge that he had not. It is lying. R. Aaron Levine (Moral Issues of the Marketplace in Jewish Law, pp. 31-35) argues that failing to properly cite sources is not just misrepresentation but also a lack of gratitude (link).
The issue of theft is a matter of debate (link). According to one view, an artist or creator possesses ownership over his creation and someone who uses it without permission is guilty of theft. According to the other view, Jewish law does not have a concept of intangible theft and only secular law prevents using a creation without permission.
In other words, there are multiple reasons that plagiarism is wrong. Lying is one of them and theft might be another.