To steal ideas from one person is plagiarism; to steal from many is research.
It's only plagiarism if you do not credit the person for their orginal idea. Every great scientist, researcher, philosopher, etc. has stood on the shoulders of others. Bohr came up with his version of the Atomic Theory--but it was nt entirely correct. But with his knowledge and lack or present day resources, it is amazing that he could even propose such a theory. Other scientists merely clarified and/or expanded on his orginal theory (EX. Electrons do not travel in perfect orbitals as Bohr theorized but rather orbitals: "clouds" or areas where an electron is most likely to be found--in simplistic terms.)
In art, beginners are told to copy copy copy the Masters are much as possible. If you cannot solve a problem (i.e. foreshorttening the forearm orf a man.) we are taught to look to other artist's work. For certainly, da Vinci had solved that problem before--foreshotening limbs. So in order to learn and make great work, present day artists stand on the shoulders of the Masters--which is why art history is so important. Why waste your time trying to slove a problem when someone else has previously solved it for you? Visiting art galleries, and certainly the Louve, you will see artists copying the works of the masters.
So long as you cite your sources, copying/using the work/data of others is essential to the learning process and to the process of building on to already claimed theories (i.e. the Atomic Theory)